Homoproline tetrazole

The conjugate addition of tetrahydrothiopyran-4-one to l-nitro-2-(3-nitrophenyl)ethene is promoted by the enantiomers of 5-pyrrolidin-2-yl-17/-tetrazole and gives the adducts in acceptable yields and with ca. 70% ee as determined by chiral HPLC (Equation 27) <20050BC84>. An enantiomeric excess of 90% was achieved using a homoproline tetrazole catalyst (Equation 28) <2005SL611>. 

Ley and coworkers also used tetrazole 5a to catalyse the asymmetric Michael addition of a ketone to an aromatic nitro-olefin in a 1 1 mixture of ethanol and isopropanol. The products obtained had moderate to good diastereoselectivities (up to >19 1 dr) and moderate enantioselectivities (up to 73% ee). Further, a homoproline tetrazole derivative (5b) was prepared and used for asymmetric Michael reaction. Catalysts 5a and 5b gave similar diastereoselectivities however, the catalyst 5b produced products with higher enantioselectivities in the Michael addition of ketones to aromatic nitro-olefins (Scheme 9.34). Ley and coworkers explained that the side chain of the homotetrazole was responsible for the increased enantioselectivity. 

P-protonation 511 homogeneous catalysts 331 homogenous a-alkylation/allylation 726 homoproline tetrazole 1015... 

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