Dichloro[TADDOLato(2−)‐ O , O ′]titanium/Dichlorobis[1‐methylethoxy]titanium‐Mediated, Highly Diastereo‐ and Enantioselective Additions of Silyl Enol Ethers to Nitro Olefins and [3+2] Cycloadditions of Primary Adducts to Acetylenes

The diastereoselective, Ti‐ Lewis ‐acid‐mediated, low‐temperature addition of silyl enol ethers to 1‐aryl‐2‐nitroethenes ( Scheme 1 ) occurs enantioselectively with dichloro[TADDOLato(2−)‐O,O′]titanium 3 (TADDOL= α , α , α ′, α ′‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanol) ( Scheme 2 ). At least 3 equiv. of Lewis acid are required for high conversions (yields). However, the chiral Lewis acid 3 can be `diluted' with the achiral Cl 2 Ti(OCHMe 2 ) 2 analog (ratio 1 : 2.5), with hardly any loss of enantioselectivity! Both, the primary (4+2) cycloadducts ( B , 9 ) and the γ ‐nitro ketones ( A , 1a  –  h , 5 , 7 ), formed by hydrolysis, can be isolated in good yields and with high configurational purities ( Schemes 3 and 4 , and Table 1 ). The relative and absolute configurations (2 S ,1′ R ) of the products 1 from cyclohexanone silyl enol ether and 1‐aryl(including 1‐heteroaryl)‐2‐nitroethenes (obtained with ( R , R )‐TADDOLate) are assigned by NMR spectroscopy, and optical comparison and correlation with literature data, as well as by anomalous‐dispersion X‐ray crystal‐structure determination (nitro ketone 1c ; Fig .). The nitro ketone 7 from cyclohex‐2‐enone and 4‐methoxy‐ β ‐nitrostyrene was cyclized ( via a silyl nitronate C ; Scheme 5 ) to the nitroso acetal 8 , and one of the bicyclic nitronate primary adducts 9 underwent a [3+2] cycloaddition to phenylacetylene and to ethyl 2‐butynoate to give, after a ring‐contracting rearrangement, tricyclic aziridine derivatives with five consecutive stereocenters ( 10 , 11 ; Scheme 5 and Table 2 ), in enantiomerically pure form. With an aliphatic nitro olefin, the Ti‐TADDOLate‐mediated reaction with (silyloxy)cyclohexene led to a moderate yield, but the product 4 was isolated in a high configurational purity.