Trimethylsilyl hetero Diels-Alder reaction

Tocopheryl)propionic acid (50) is one of the rare examples that the o-QM 3 is involved in a direct synthesis rather than as a nonintentionally used intermediate or byproduct. ZnCl2-catalyzed, inverse hetero-Diels-Alder reaction between ortho-qui-none methide 3 and an excess of <2-methyl-C,<9-bis-(trimethylsilyl)ketene acetal provided the acid in fair yields (Fig. 6.37).67 The o-QM 3 was prepared in situ by thermal degradation of 5a-bromo-a-tocopherol (46). The primary cyclization product, an ortho-ester derivative, was not isolated, but immediately hydrolyzed to methyl 3-(5-tocopheryl)-2-trimethylsilyl-propionate, subsequently desilylated, and finally hydrolyzed into 50. 

Hetero-Diels-Alder Reactions of Ketones. Ketonic substrates such as ethyl pyruvate (eq 9, R =Me, R = OEt) do not react with simple dienes such as cyclopentadiene or 1,3-cyclohexadiene in the presence of (5, 5)-/-Bu-box and a metal salt as catalyst. However, using activated dienes such as /ranf-l-methoxy-3-[(trimethylsilyl)oxyl]-l,3-butadiene (Danishefsky s diene), a hetero-Diels-Alder reaction with ethyl pyruvate and similar substrates catalyzed by 10 mol% of Cu[(5, 5)-/-Bu-box] (OTf)2 takes place in good yields and enantioselec-tivities (eq 9). Surprisingly, it was even possible to reduce the catalyst loading to only 0.5 mol % without affecting the yield of the product, and in some cases the enantiomeric excess was even improved. 

In the synthesis of analogs of the antibiotic spectinomycin, Danishefsky and coworkers used the hetero Diels-Alder reaction to construct a precursor to install the stereochemistry on the tricyclic ring system. " Trimethylsilyl diene (145) reacts with acetaldehyde and a catalytic amount of Eu(fod)3 to give a 5.7 1... 

Hetero-Diels-Alder reaction. 3,4-Bis(trimethylsilyl)-5,6-dihydro-2A/-pyrans are accessible from a catalyzed cycloaddition under very mild conditions. The diene is prepared in 44-47% yield from the reaction of l,4-ditosyloxy-2-butyne with trimethylsilylmethylmagnesium chloride. 

The catalytic enantioselective hetero-Diels-Alder reactions between ketomalonate and activated and non-activated conjugated dienes yield cycloadducts which can be converted into chiral COa-synthons. The intramolecular Diels-Alder reaction of sulfolene masked diene synthon (169) yield 5-carbomethoxy yohimbine systems (170) and (171) with ejco-stereochemistry (Scheme 65). Trimethylsilyl triflate catalyses... 

Tridentate salen ligands (10) derived from 1 have given excellent results in the enantiocontrol of the hetero Diels-Alder addition reaction of dienes with aldehydes (eq 7) and in the asymmetric additions of TMS-azide to mc5o-epoxide and trimethylsilyl cyanide to benzaldehyde (up to 85% ee). Phosphino-oxazolines derived from 1 have been employed for the asymmetric control of palladium-catalyzed allylic substitution reactions products of 70-90% ee were obtained. Photolysis of crystalline adducts of enantiomerically pure 1 with prochiral alcohols results in asymmetric inductions of up to 79% in a rare example of a solid-state enantioselective reaction. ... 

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