Tandem reactions enolate-trapping cyclization

The key features of the catalytic cycle are trapping of the radical generated after cycliza-tion by an a,P-unsaturated carbonyl compound, reduction of the enol radical to give an enolate, and subsequent protonation of the titanocene alkoxide and enolate. The diaster-eoselectivity observed is essentially the same as that achieved in the simple cyclization reaction. An important point is that the tandem reactions can be carried out with alkynes as radical acceptors. The trapping of the formed vinyl radical with unsaturated carbonyl compounds occurs with very high stereoselectivity, as shown in Scheme 12.21. 

The y-lactam 110 is prepared by the reaction of the lithium silyl-substituted ynolate 105 with the aziridine 108 activated by a p-toluenesulfonyl group. The initial product is the enolate 109, which can be acidified to yield the a-silyl-y-lactam 110. Intermediate 109 can be trapped by aldehydes to afford the a-alkylidene-y-lactams 111 via a Peterson reaction (equation 45) . These reactions may be considered to be formal [3 + 2] cycloadditions as well as tandem reactions involving nucleophilic ring opening and cyclization. 

A mixture of THF and DMPU has been used as a solvent for Sml2 in various reactions such as cyclization of alkynyl halides [90,91], tandem iodo-enone cy-clization/samarium enolate aldol reaction [92], coupling of (3-silylacrylic esters [93], deprotection of arenesulfonamides [94] and pyridine-2-sulfonamides [95], radical ring-opening reactions of cyclopropyl ketones and the trapping of the resulting samarium(III) enolates by a variety of electrophiles [96] (Scheme 41). 

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