Cyclization destannylative

The synthesis of different substituted finans by cyclization of 4-pentynones using potassium tert-butoxide in DMF was reported <96TL3387>. Dihydrofuran 32 can be prepared by a destannylative acylation of l-[(2-methoxyethoxy)methoxy]-2-(phenylsulfonyl)-2-(tributylstannyl)-cyclopropane. Treatment of 32 with BFj-EtjO yields 3-acyUurans via an intramolecular Prins-type reaction of the resulting oxonium ion intermediate <96TL4585>. 

Entries 20 to 23 involve additions to C=N double bonds in oxime ethers and hydrazones. These reactions result in installation of a nitrogen substituent on the newly formed rings. Entry 20 involves the addition of the triphenylstannyl radical to the terminal alkyne followed by cyclization of the resulting vinyl radical. The product can be proto-destannylated in good yield. The ring closure generates an anti relationship for the amino substituent, which is consistent with the TS shown below. 

Tandem addition of trimethyltin followed by cyclization in a 1,6-heptadiene system [Eq, (7)] proceeds with surprising efficiency [30]. Oxidative destannylation of the primary product gives a synthetically useful dimethyl acetal. An acetylene-terminated tandem addition is shown in Eq. (8) [31]. 

All the examples described above involved the reaction of diazoacetate derivatives with silver salts to initiate the formation of a putative silver carbene however, other pathways exist. For example, Porcel and Echavarren have reported an intramolecular cyclization of an allylstannane to a pendent alkyne (Scheme 8.22) that involves the intermediacy of a silver carbene.52 As can be seen in Table 8.12, the reactions proceeded in moderate to excellent yield, providing the dienylstannane, while in some cases, reductive destannylation occurred. Several asymmetric reactions were reported with substrate ( )-145d, leading to the formation of the expected adduct in reasonable enantioselectivities (ee = 73-78%) in a preliminary screen with a number of different ligands. 

Insertion reactions using CO and CH2N,2. Radical cyclization reactions of azaenynes in the presence of butyl stannanes in carbonylation conditions furnished /3-lactams <2003JA5632>. Azocanone 150 (Scheme 62) was prepared in good yield from enyne 149. The reaction occurred via a-stannylmethylene lactam of type 154 (Scheme 63). This free radical-mediated stannyl carbonylation is quite versatile and provides a general [ -pl]-type annulation leading to 4-exo and 8-exo systems. The yield of the isolated stannylene lactam intermediate 154 was reported as 61%, while the destannylation to lactam 150 was quantitative. 

S-Lactones. Oxidative cyclization of 3-hydroxy-5-tributylstannylpentanoic acids with Pb(OAc)4 results in destannylation and lactone formation. a-Substituted 8-lactones are obtained when alkanols undergo oxidation under CO (20 examples, 32-15%) ... 

Use of cyclization for access to optically active heterocycies has been studied extensively (Scheme 12.65) [137]. When an enantiomerically defined homoallyloxy stannane was treated with BuLi the desired cychzation occurred to afford the optically active furan derivative and the destannylated compound. Addition of LiCI to this cyclization procedure enhanced the yield to furnish trans and cis products in high enantio excess. 

The y-stannylcarbenium ion formed is stabilized by homoconjugation of the carbon-tin bond and undergoes destannylative cyclization leading to cyclopropane (25) (Scheme 9.22). In the presence of (la), N,N-bis(siloxy)enamines, prepared by double silylation of nitroalkanes, react with acetals to give l-alkoxy-3-nitroalkanes [76]. 

In the total synthesis of the hesitine-type aconite alkaloid nominine, the C-ring was constructed by radical cyclization to form 118 from the enyne precursor 117. ° In the first step, the tin radical reacted with alkyne 117 to form a radical that underwent a 6-exo-trig radical cyclization. The resulting radical was trapped by BuaSnH to give a vinyl stannane, which afforded 117 after destannylation with SiOi (Scheme 25.56). 

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