Cycloaddition reactions chiral sulfinyl groups

Sulfinyl dienes and vinyl sulfoxides have rarely been used in asymmetric hete-ro-Diels-Alder reactions [145]. The first example was reported in 1992 and describes an intramolecular cycloaddition using a heterodiene bearing a chiral sulfinyl group [146a]. In this paper, the conversion of a-p-tolylsulfinyl a,ft-unsaturated ketone 176 (prepared by Knoevenagel reaction of 3-methylcitronellal and (S)-p-toluenesulfinylacetone) into the hetero-Diels-Alder adducts 177... 

Reactions of 3,5-dichloro-2,4,6-trimethyl benzonitrile oxide 241 with fluoro-methyl substituted alkenes 242, bearing a chiral sulfinyl group at -position of the double bond, afford diastereoisomeric 4,5-dihydroisoxazoles 243 and 244 [180] with a stereoselectivity lower than 2 1 (Scheme 110). The authors conclude that the efficiency of allyl sulfoxides to control diastereoselectivity of 1,3-dipolar cycloadditions with nitrile oxides is lower than that of vinyl sulfoxides. 

Asymmetric [4 + 2] cycloadditions of an optically active diene, or dienophile with chiral sulfinyl groups, with a quaternary sulfur center have succeeded in the LiC104-CH2CI2 medium. When chiral dienophile 66 and CP were subjected to cyclization, neither the endotexo ratio nor the enantiofacial selectivity of endo adducts was high [83], In the reaction of chiral diene 67 with MA, the best catalyst was LiC104-CH2CI2 this gave only endo isomers endo-6Sa and endo-6Sb in 70 % yield in the ratio of 96 4, presumably via TS-2 (Sch. 35) [84]. 

It has been shown that the sulfinyl group present as chiral auxiliary either in dienophiles or in dienes is very useful for controlling the enantio- and diastereofacial selectivity in the asymmetric Diels-Alder reaction [43]. A wide variety of enantiomerically pure cyclohexadienedicarboxylates has been produced by cycloaddition of the sulfinylmaleate 39 with several dienes under catalyzed... 

The last problem associated with the use of the sulfinyl group as a chiral inductor in cycloaddition reactions is related to the endo/exo reactivity. From the available data, the endo directing power of the sulfinyl group is not large. The need to incorporate other functional groups into vinyl sulfoxides, thus increasing their dienophilic reactivity, decrees that the formation of mixtures is difficult to avoid in many cases as a consequence of the competition between the... 

The Weinreb group has recently examined the reaction of chiral N-sulfinyl dienophile 23, prepared from (+)-camphor, with 1,3-cyclohex-adiene (Scheme 1-VlII). Whereas the uncatalyzed cycloaddition afforded a mixture of diastereomeric adducts, the reaction promoted by TiCU gave a single adduct 24 having the 35,6ft configuration. Stereochemistry at sulfur in this compound could not be determined. As in the phenylmenthol series, one can reasonably consider two reacting dienophile conformations 23A and 23B (Scheme 1-VlII). If conformer 23A is attacked by the diene in an endo manner from the most exposed face, the observed adduct 24 will be formed. Similarly, if conformer 23B reacts with cyclohexadiene via an exo transiticm state, 24 wiU result. 

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