WO 2006/116150 PCT/US2006/015192 DIHYDROBENZOFURAN DERIVATIVES AND USES THEREOF CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application claims priority to United States Provisional Patent Application serial number 60/674,150, filed April 22, 2005, the entirety of which is hereby incorporated herein by reference. FIELD OF THE INVENTION [0002] The present invention relates to 5-HT2C receptor agonists, processes for their preparation, and uses thereof. BACKGROUND OF THE INVENTION [0003] Schizophrenia affects approximately 5 million people. The most prevalent treatments for schizophrenia are currently the 'atypical' antipsychotics, which combine dopamine (D2) and serotonin (5-HT2A) receptor antagonism. Despite the reported improvements in efficacy and side- effect liability of atypical antipsychotics relative to typical antipsychotics, these compounds do not appear to adequately treat all the symptoms of schizophrenia and are accompanied by problematic side effects, such as weight gain (Allison, D. B., et. al., Am. J. Psychiatry, 156: 1686-1696, 1999; Masand, P. S., Exp. Opin. Pharmacother. I: 377-389, 2000; Whitaker, R., Spectrum Life Sciences. Decision Resources. 2:1-9, 2000). [0004] Atypical antipsychotics also bind with high affinity to 5-HT2C receptors and function as 5-HT2C receptor antagonists or inverse agonists. Weight gain is a problematic side effect associated with atypical antipsychotics such as clozapine and olanzapine, and it has been suggested that 5-HT2C antagonism is responsible for the increased weight gain. Conversely, stimulation of the 5-HT2C receptor is known to result in decreased food intake and body weight (Walsh et. al., Psychopharmacology 124: 57-73, 1996; Cowen, P. J., et. al., Human Psychopharmacology 10: 385-391,1995; Rosenzweig-Lipson, S., et. al., ASPET abstract, 2000). [0005] Several lines of evidence support a role for 5-HT2C receptor agonism or partial agonism as a treatment for schizophrenia. Studies suggest that 5-HT2C antagonists increase synaptic levels of dopamine and may be effective in animal models of Parkinson's disease (Di Matteo, V., et. al., Neuropharmacology 37: 265-272, 1998; Fox, S. H., et. al., Experimental Neurology 151: 35-49, 1998). Since the positive symptoms of schizophrenia are associated with 1 WO 2006/116150 PCT/US2006/015192 increased levels of dopamine, compounds with actions opposite to those of 5-HT2C antagonists, such as 5-HT2C agonists and partial agonists, should reduce levels of synaptic dopamine. Recent studies have demonstrated that 5-HT2C agonists decrease levels of dopamine in the prefrontal cortex and nucleus accumbens (Millan, M. J., et. al., Neuropharmacology 37: 953-955,1998; Di Matteo, V., et. al., Neuropharmacology 38: 1195-1205, 1999; Di Giovanni, G., et. al., Synapse 35: 53-61, 2000), brain regions that are thought to mediate critical antipsychotic effects of drugs like clozapine. However, 5-HT2C agonists do not decrease dopamine levels in the striatum, the brain region most closely associated with extrapyramidal side effects. In addition, a recent study demonstrates that 5-HT2C agonists decrease firing in the ventral tegmental area (VTA), but not in the substantia nigra. The differential effects of 5-HT2C agonists in the mesolimbic pathway relative to the nigrostriatal pathway suggest that 5-HT2C agonists have limbic selectivity, and will be less likely to produce extrapyramidal side effects associated with typical antipsychotics. SUMMARY OF THE INVENTION [0006] The present invention relates to 5-HT2C agonists and uses thereof. The compounds of the present invention are useful, for example, to treat schizophrenia and the concomitant mood disorders and cognitive impairments of schizophrenia. In certain embodiment, compounds of the present invention are less likely to produce the body weight increases associated with current atypical antipsychotics. The compounds of the present invention are also useful for the treatment of obesity and its comorbidities. [0007] In certain embodiments, the present invention provides a compound of formula I: or pharmaceutically acceptable salts thereof, wherein: each =----= independently represents a single or double bond, provided that both == groups do not simultaneously represent a double bond; R1 is hydrogen, lower alkyl, or -C(O)R; R is hydrogen or lower alkyl; R2 is hydrogen, -OH, or -OS(O)2OH; 2 WO 2006/116150 PCT/US2006/015192 R3 is hydrogen, halogen, methyl, methoxy, -OH, or-OS(0)2OH; R4 is hydrogen, -OH, or -OS(O)2OH; Ra and Rb are each hydrogen or are taken together to form an oxo moiety; Rc and Rd are each hydrogen or are taken together to form an oxo moiety; each Rx is independently halogen or lower alkyl; each Ry is independently hydrogen, -OH, or -OS(O)2OH, wherein at least one of R2, R4, and Ry is -OH or-OS(O)2OH;and n is one or two. [0008] In certain other embodiments, the invention relates to methods for treating a patient suffering from schizophrenia, sehizophreniform disorder, schizoaffective disorder, delusional disorder, substance-induced psychotic disorder, L-DOPA-induced psychosis, psychosis associated with Alzheimer's dementia, psychosis associated with Parkinson's disease, psychosis associated with Lewy body disease, dementia, memory deficit, intellectual deficit associated with Alzheimer's disease, bipolar disorders, depressive disorders, mood episodes, anxiety disorders, adjustment disorders, eating disorders, epilepsy, sleep disorders, migraines, sexual dysfunction, substance abuse, addiction to alcohol and various other drugs, including cocaine and nicotine, gastrointestinal disorders, obesity, or a central nervous system deficiency associated with trauma, stroke, or spinal cord injury that includes administering to the patient a therapeutically effective amount of a compound of formula I, or a pharmaceutically acceptable salt thereof. [0009] In still other embodiments, the invention relates to compositions comprising a compound of formula I or a pharmaceutically acceptable salt thereof, and one or more pharmaceutically acceptable carriers, excipients, or diluents. DETAILED DESCRIPTION OF THE INVENTION 1. Compounds and Definitions: [0010] The present invention relates to compounds that are agonists or partial agonists of the 2C subtype of brain serotonin receptors. [0011] The term "lower alkyl," as used herein, refers to a hydrocarbon chain having up to 4 carbon atoms, preferably 1 to 3 carbon atoms, and more preferably 1 to 2 carbon atoms. The term "alkyl" includes, but is not limited to, straight and branched chains such as methyl, ethyl, n- propyl, isopropyl, n-butyl, isobutyl, sec-butyl, or t-butyl. [0012] The terms "halogen" or "halo," as used herein, refer to chlorine, bromine, fluorine or iodine. 3 WO 2006/116150 PCT/US2006/015192 [0013] The terms "effective amount" and "therapeutically effective amount," as used herein, refer to the amount of a compound of formula I that, when administered to a patient, is effective to at least partially treat a condition from which the patient is suffering from. Such conditions include, but are not limited to, schizophrenia, schizoaffective disorder, schizophreniform disorder, L-DOPA-induced psychosis, bipolar disorder, obesity, obsessive compulsive disorder, depression, panic disorder, sleep disorders, eating disorders, and epilepsy. [0014] The term "pharmaceutically acceptable salts" or "pharmaceutically acceptable salt" refers to salts derived from treating a compound of formula I with an organic or inorganic acid such as, for example, acetic, lactic, citric, cinnamic, tartaric, succinic, fumaric, maleic, malonic, mandelic, malic, oxalic, propionic, hydrochloric, hydrobromic, phosphoric, nitric, sulfuric, glycolic, pyruvic, methanesulfonic, ethanesulfonic, toluenesulfonic, salicylic, benzoic, or similarly known acceptable acids. In certain embodiments, the present invention provides the hydrochloride salt of a compound of formula I. [0015] The term "patient," as used herein, refers to a mammal. In certain embodiments, the term "patient", as used herein, refers to a human. [0016] The terms "administer," "administering," or "administration," as used herein, refer to either directly administering a compound or composition to a patient, or administering a prodrug derivative or analog of the compound to the patient, which will form an equivalent amount of the active compound or substance within the patient's body. [0017] The terms "treat" or "treating," as used herein, refers to partially or completely alleviating, inhibiting, preventing, ameliorating and/or relieving the condition. [0018] The terms "suffer" or "suffering" as used herein refers to one or more conditions that a patient has been diagnosed with, or is suspected to have. 2. Description of Exemplary Compounds: [0019] In certain embodiments, the invention relates to a compound of formula I: 4 WO 2006/116150 PCT/US2006/015192 or pharmaceutically acceptable salts thereof, wherein: each == independently represents a single or double bond, provided that both == groups do not simultaneously represent a double bond; R1 is hydrogen, lower alkyl, or -C(O)R; R is hydrogen or lower alkyl; R2 is hydrogen, -OH, or -OS(O)2OH; R3 is hydrogen, halogen, methyl, methoxy, -OH, or -OS(O)2OH; R4 is hydrogen, -OH, or -OS(O)2OH; Ra and Rb are each hydrogen or are taken together to form an oxo moiety; Rc and Rd are each hydrogen or are taken together to form an oxo moiety; each Rx is independently halogen or lower alkyl; each Ry is independently hydrogen, -OH, or -OS(O)2OH, wherein at least one of R2, R4, and Ry is -OH or-OS(O)2OH;and n is one or two. [0020] As defined generally above, each == independently represents a single or double bond, provided that both == groups do not simultaneously represent a double bond; Ra and Rb are each hydrogen or are taken together to form an oxo moiety; and Rc and Rd are each hydrogen or are taken together to form an oxo moiety. It will be appreciated that when a == group represents a double bond, then the corresponding Ra and Rb or Rc and Rd pair must represent a single hydrogen. Accordingly, compounds of formula I are contemplated such that valency permits. [0021] In certain embodiments, the n group of formula I is 1. [0022] In other embodiments, the n group of formula I is 2. [0023] As defined generally above, the R1 group of formula I is R1 is hydrogen, lower alkyl, or -C(O)R, wherein R is hydrogen or lower alkyl. In certain embodiments, the R1 group of formula I is hydrogen. In other embodiments, the R1 group of formula I is lower alkyl. In still other embodiments, the R1 group of formula I is methyl. According to another aspect, the R1 group of formula I is -C(O)R wherein R is hydrogen or methyl. [0024] In certain embodiments, the R3 group of formula I is fluoro or chloro. In other embodiments, the R3 group of formula I is fluoro. [0025] In certain embodiments, the R2 group of formula I is -OH and the R4 group is hydrogen. In other embodiments, the R2 group is hydrogen and the R4 group of formula I is -OH. In still other embodiments, both of the R2 and R4 groups of formula I are -OH. 5 WO 2006/116150 PCT/US2006/015192 [0026] In certain embodiments, eachRx group of formula I is independently halogen or methyl. In other embodiments, each Rx group of formula I is independently a halogen group. In still other embodiments, both Rx groups of formula I are chloro. [0027] In certain embodiments, at least one of the R2, R4, and Ry groups of formula I is -OS(O)2OH. According to another aspect, the present invention provides a compound of any of formulae I-a, I-b, I-c, I-d, and I-e: or pharmaceutically acceptable salts thereof, wherein each of R2, R4, Rx, and Ry is as defined above and in classes and subclasses described herein. [0028] In certain embodiments, the == moiety of formula I is a double bond. In other embodiments, the present invention provides a compound of formula I-f: 6 WO 2006/116150 PCT/US2006/015192 or pharmaceutically acceptable salts thereof, wherein each of R2, R3, R4, Rx, and Ry is as defined above and in classes and subclasses described herein. [0029] According to one aspect of the present invention, Ra and Rb are taken together to form an oxo moiety. According to another aspect, Rc and Rd are taken together to form an oxo moiety. In certain embodiments, the present invention provides a compound of formula I-g or I-h: or pharmaceutically acceptable salts thereof, wherein each of R2, R3, R4, Rx, and Ry is as defined above and in classes and subclasses described herein. [0030] In other embodiments, the present invention provides a compound of formula I': or pharmaceutically acceptable salts thereof, wherein each of R1, n, R2, R3, R4, Rx, and Ry is as defined above and in classes and subclasses described herein. [0031] According to another embodiment, the present invention provides a compound of formula II: 7 WO 2006/116150 PCT/US2006/015192 or pharmaceutically acceptable salts thereof, wherein each Rx, Ry, and R4 are as defined generally above and in classes and subclasses described above and herein. [0032] According to yet another embodiment, the present invention provides a compound of formula III: or pharmaceutically acceptable salts thereof, wherein each Rx, Ry, and R2 are as defined generally above and in classes and subclasses described above and herein. [0033] According to yet another embodiment, the present invention provides a compound of formula IV: or pharmaceutically acceptable salts thereof, wherein each Rx, R2 and R4 are as defined generally above and in classes and subclasses described above and herein. [0034] According to yet another embodiment, the present invention provides a compound of formula V: or pharmaceutically acceptable salts thereof, wherein each Rx, R2 and R4 are as defined generally above and in classes and subclasses described above and herein. 8 WO 2006/116150 PCT/US2006/015192 [0035] according to yet another embodiment, the present invention provides a compound of formula VI: or pharmaceutically acceptable salts thereof, wherein each Rx, R2 and R4 are as defined generally above and in classes and subclasses described above and herein. [0001] Compounds of the present invention contain asymmetric carbon atoms and thus give rise to stereoisomers, including enantiomers and diastereomers. Accordingly, it is contemplated that the present invention relates to all of these stereoisomers, as well as to mixtures of the stereoisomers. Throughout this application, the name of the product of this invention, where the absolute configuration of an asymmetric center is not indicated, is intended to embrace the individual stereoisomers as well as mixtures of stereoisomers. In certain embodiments of the invention, compounds having an absolute (R) configuration are preferred. [0002] In certain embodiments, the present invention provides a compound of formula Ia or Ib: or a pharmaceutically acceptable salt thereof, wherein each R1, R2, R3, R4, Rx and Ry are as defined above for compounds of formula I and in classes and subclasses as described above and herein. [0003] According to another embodiment, the present invention provides a compound of formula Ic or Id: 9 WO 2006/116150 PCT/US2006/015192 or a pharmaceutically acceptable salt thereof, wherein each R1 , R2, R4 , Rx, and Ry are as defined above for compounds of formula I and in classes and subclasses as described above and herein. [0004] Where an enantiomer is preferred, it may, in some embodiments be provided substantially free of the corresponding enantiomer. Thus, an enantiomer substantially free of the corresponding enantiomer refers to a compound which is isolated or separated via separation techniques or prepared free of the corresponding enantiomer. "Substantially free," as used herein, means that the compound is made up of a significantly greater proportion of one enantiomer. In certain embodiments the compound is made up of at least about 90% by weight of a preferred enantiomer. In other embodiments of the invention, the compound is made up of at least about 99% by weight of a preferred enantiomer. Preferred enantiomers may be isolated from racemic mixtures by any method known to those skilled in the art, including chiral high pressure liquid chromatography (HPLC) and the formation and crystallization of chiral salts or prepared by methods described herein. See, for example, Jacques, et al., Enantiomers, Racemates and Resolutions (Wiley Interscience, New York, 1981); Wilen, S.H., et al., Tetrahedron 33:2725 (1977); Eliel, E.L. Stereochemistry of Carbon Compounds (McGraw-Hill, NY, 1962); Wilen, S.H. Tables of Resolving Agents and Optical Resolutions p. 268 (E.L. Eliel, Ed., Univ. of Notre Dame Press, Notre Dame, IN 1972). [0036] It is recognized that atropisomers of the present compounds may exit. The present invention thus encompasses atropisomeric forms of compounds of formula I as defined above, and in classes and sublcasses described above and herein. [0037] Exemplary compounds of formula I are set forth in Table 1, below. 10 WO 2006/116150 PCT/US2006/015192 Table 1: Exemplary Compounds of Formula I: 11 WO 2006/116150 PCT/US2006/015192 2006/116150 PrT/m™n*/nisi