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 dichloroTADDOLato(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 Cl₂Ti(OCHMe₂)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 (2S,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.