Stannylated enynes

The syn stereoselectivity exhibited the stannylcupration of alkynes is used for the synthesis of stannylated enynes 139 which could be readily transformed to the desired enediynes [67] [Eq. (46)]. 

Stannaboration of diynes and enyne provide cycloalkanes incorporating both stannyl and boryl groups, which are otherwise difficult to synthesize. Palladium-catalyzed reactions of 1,4-, 1,5-, and 1,6-diynes with 9 give the corresponding cyclization products in good yields (Equation (99)).247 The reaction is so efficient that even a strained four-membered ring is readily formed. 

Mechanistically, a (silyl)(stannyl)palladium initially formed undergoes regioselective silylpalladation to the alkyne moiety of the enyne (Scheme 66). Then, two possible pathways are conceivable for addition to the alkene moiety, that is, stannylpalladation and carbopalladation. It has not been established that which pathway operates. 

The western part 97 of tylosin aglycon (96), a 16-membered macrolide, has also been synthesized using this Kocienski metalate rearrangement [66]. Treatment of the lithiated dihydrofuran 99 with the stannyl cuprate [67] obtained from Bu3SnLi and CuCN, followed by Mel alkylation, exclusively gave the E vinyl stan-nane 100, in 80% yield. In the last stage, stannyl cupration [68] of the deprotected enyne diol 101 afforded the desired ( , E) stannyl diene 97 in 85% yield. 

The addition of Cp2Zr(H)Cl, known as the Schwartz reagent [30], to different alkenes and alkynes is known to be a facile process [31]. Therefore, the hydrozirconation of a variety of readily available enynes 12 is among the first methods developed for the stereoselective preparation of dienyl zirconium reagents 13. This process is both completely chemo- and regioselective with a syn addition of the zirconium hydride across the alkyne [32] (Scheme 5). From the same intermediate, the Zr atom can be isomerized in its internal position such as in 15 via a zirconacyclopropene intermediate 14. Moreover, the addition of trimethylstannyl chloride to 14 led to the stannylated dienyl zirconocene 16 [33] (Scheme 5). 

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. 

First the enyne alcohol is converted into the TBS ether via step 1. For introduction of the stannane moiety the alkync is deprotonated by a base, in this case by KHMDS, followed by addition of Bu SnCI. Cis selective reduction of the resulting stannylalkyne is achieved via hydrozireonation using equimolar amounts of CpoZrHCl. This method is not very common and only applied to stannyl alkynes with ether functionalities. Limitation of this method is related to the... 

Interesting compounds that have been acquired from the metathesis process include bicyclic P-lactams (also those comprising a diene unit by the enyne metathesis ), a-stannyl cyclic ethers, cyclic siloxanes which can be opened to afford (Z)-alkenediols, spiroannulated dihydrofurans and dihydropyrans, and [2]catenanes from precursors organized by Cu(I). ... 

The free-radical cyclization of enynes as a synthetic strategy was introduced by Stork and coworkers using BusSnH/AIBN-mediated ethynyl cyclization (equation 34) ". From a mechanistic point of view, the first step is the selective addition of the stannyl radical to the triple bond of the enyne generating a vinyl radical, which can cyclize onto the double bond. In the absence of the methyl substituents on the olefinic moiety, the groups of Beckwith and Stork have both postulated, and demonstrated with the aid of kinetic studies, that the 6-endo radical 30 arises from a ring expansion of the kinetically favored... 

SCHEME 14.12 Stannylative Pd-catalyzed [4 -I- 2] cross-benzannulation reaction of two enynes. 

In a manner analogous to the palladium-catalyzed [4 -l- 2] benzannulation reaction of two enynes, the stannylative version of the benzannulation reaction of enyne with diyne was also successful, providing 3-alkynylarylstannane 40 in 85% yield [10] (Scheme 14.18). Notably, the metallative version of this reaction made it possible to introduce substitution at the mefa-position to the alkynyl moiety, which is the only nonfunctionalizable position via the nonmetallative method. 

The octaene 1 was assembled from four stereodefined fragments. The first, the linchpin 6, was prepared from the stannyl aldehyde 4. Homologation gave the enyne 5, which on hydroboiation and oxidation gave 6. 

Stannyl enol ethers react with electron-deficient alkenes like or, -unsaturated esters and alkynes in the presence of AIBN to give radicalic carbostannylation adducts, y-stannyl ketones, " " in sharp contrast to the Bu4NBr-mediated reaction, which gives simple Michael adducts. In addition, 1,6-enynes, when applied to this carbostannylation, produce five-membered cyclic adducts. For this particular transformation, electron-deficient groups are not needed (Scheme 3-222). 

Nakao, Y., Hirata, Y., Ishihara, S. et al. (2004) Stannylative cycloaddition of enynes catalyzed by palladium-iminophosphine. Journal of the American Chemical Society, 126(48), 15650-15651. 

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