7-Alkoxy allylic stannanes reactions

Double asymmetric reactions between [7-(alkoxy)allyl]stannanes 230 and the a-benzyloxy aldehyde 55 exhibited clear matched and mismatched behavior [168]. With BF3 OEt2 catalysis, the matched double asymmetric reaction between (R)-230a and aldehyde (S)-55 generates exclusively the syn,anti adduct 425 (Eq. (11.40)). Formation of 425 can be rationalized through either the antiperipla-nar, Felkin transition state 426 (as proposed by Marshall) or the synclinal Felkin transition state 427. 

Upon treatment with BFj OEt2, stannane 265 is isomerized via an inter-molecular pathway, resulting in allylic transposition and stereochemical inversion of configuration to yield 266 (Scheme 5.2.57, bottom), and this process provides an efficient route to non-racemic y-(alkoxy)allylic stannanes. The reaction of stannane 267 is advanced with unsaturated aldehydes and achieves facial selectivity by the antiS mechanism giving mainly the syn product 268, containing an E-alkenyl ether. The anti-SE arrangement shown in 270 minimizes non-bonded interactions leading to the major product. Similarly, the chiral stannane 271 adds to aliphatic aldehydes to produce the E-iyn-alcohol 272 (Scheme 5.2.58). ... 

Allylation reactions can be designed to effect high stereoselectivity in the case of chiral /3-alkoxy aldehydes, in which the ether oxygen provides for effective coordination with a Lewis acid. Multi-valent, oxophilic Lewis acids serve to pre-organize the aldehyde substrate in a six-membered chelation complex. As in the examples of a-chelation control, an open transition state is deployed with synchnal or antiperiplanar orientations based upon the consideration of steric interactions with placement of the small (hydrogen) vinyl substituent of the allylic stannane over the preformed metallocycle. Several examples are illustrated in Scheme 5.2.20. i... 

SET photochemistry is involved in the reaction between the enones (371) and the a-stannyl ethers (372) in methanol. The products are the 3-sub-stituted cycloalkanes (373) which arise from addition of aryloxymethyl radicals to the enones. Irradiation (X > 400 nm) of the stannanes (374) in the presence of the ketones and aldehydes (375) affords two products identified as (376) and (377). The former of these is dominant and the reaction arises by an electron transfer from the stannane to the ketone. The resultant stannane radical-cation undergoes fission to yield an alkoxy allyl radical and the tin cation. The alkoxyalkyl radical adds to the carbonyl radical-anion with a preference for... 

A number of investigations have explored the reactions of ally lie stannanes containing a y-alkoxy substituent. A direct preparation of these substances utilizes the kinetic deprotonation of an allyl ether followed by alkylation with tri-n-butylstannyl chloride. In a typical experiment, the deprotonation of 101 with 5-butyllithium leads to internal coordination of lithium cation and provides formation of the Z-allylstannane 102. The behavior of y-alkoxyallylstannanes is similar to the corresponding Z-alkylstannanes, and as a result, the reaction provides a stereoselective route for the synthesis of complex diol derivatives. In the allylation of chiral aldehyde 80 with stannane 102, /l-chelation dictates face selectivity. The expected. yyn, anti-product 104 is obtained with high diastereoselection via the antiperi-planar 103, which accommodates the sterically demanding silyl (TBS) ether (Scheme 5.2.23).23... 

---