Singlet oxygen chiral auxiliaries

Chiral oxazolidine compounds have also been used as chiral auxiliaries for asymmetric Diels-Alder reactions. Adam et al.8 demonstrated the cycloaddition of optically active 2,3-dimethyl oxazolidine derivatives with singlet oxygen. As shown in Scheme 5-9, the reaction of chiral substrate 39 with singlet oxygen provides product 40 in high diastereomeric ratio. 

Control of the diastereoselectivity in the singlet oxygen ene reaction can be achieved by the use of chiral auxiliaries. Success in this field would open up promising prospects in the preparation of optically active building blocks for asymmetric synthesis, since the oxygenation reaction could be followed by the removal of the chiral auxiliary. Unfortunately,... 

SCHEME 39. Control of the diastereoselectivity in the singlet oxygen ene reaction by the use of chiral auxiliaries... 

As already hinted at above, chiral dioxetanes, obtained through the highly stereoselective [2 + 2] cycloaddition of singlet oxygen to the chiral enecarbamate, provide a convenient preparation of optically active 1,2 diols as building blocks for asymmetric synthesis (Scheme 5) . Reduction of the dioxetane 2c by L-methionine, followed by release of the oxazolidinone auxiliary by NaBH4/DBU reduction, affords the enantiomerically pure like-5 diol (for additional cases, see Table 4 in Reference 19e). 

SCHEME 40. Diastereoselectivity in the chiral-auxiliary-controlled singlet oxygen ene reaction of optically active oxazolidines... 

Replacement of the Ti in these catalysts with Zr, Hf, or Ta does not improve the chiral efficiency (5S). Dibenzyl tartramide in place of tartrates is also effective as chiral auxiliary of the Ti complex and results in high ee (59). The catalyst systems formed from Ti(0-/-C3H7)4 and the tartramide in 2 2 and 2 1 mole ratio leads to opposite enantioselec-tion as shown in Scheme 24. The ene reaction of singlet oxygen and alkenes in the presence of a Ti-DET complex has been used to prepare chiral epoxy alcohols in up to 72% ee (Scheme 25) (60). 

Singlet molecular oxygen (lC>2) is a very small electrophilic reagent that can produced by photosensitization and react easily with dienes and alkenes in [4 + or [2 + 2] cycloadditions or ene-reactions. In order to develop synthetic applic tions of photooxygenations, numerous studies have recently been developed wi chiral substrates and especially in the presence of chiral auxiliaries. 

Adam W, Bosio SG, Turro NJ (2002) Flighly diastereoselective dioxetane formation in the photooxygenation of enecarbamates with an oxazolidinone chiral auxiliary steric control in the [2+2] cycloaddition of singlet oxygen through conformational alignment. J Am Chem Soc 124 8814-8815... 

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