7-Alkoxy allylic stannanes

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. 

An alternative route to nonracemic a-alkoxy stannanes entails the reduction of acyl stannanes with chiral hydrides61 62. Accordingly, conjugated stannyl enones yield (S)-a-alkoxy allylic stannanes by reduction with (J )-(+)-BINAL-H. As expected, (S)-(—)-BINAL-H gives rise to the enantiomeric (7 )-a-alkoxy allylic stannanes (equation 29)61. Upon treatment with Lewis acids, these stannanes undergo a stereospecific anti 1,3-isomerization to the (Z)-y-alkoxy allylic stannanes61. 

Marshall, J A, Seletsky, B M, Luke, G P, Synthesis of protected carbohydrate derivatives through homologation of threose and erythrose derivatives with chiral y-alkoxy allylic stannanes, J. Org. Chem., 59, 3413-3420, 1994. 

In a demonstration of the synthetic utility of chiral y-alkoxy stannane reagents, Marshall and co-workers applied this methodology to the synthesis of the gypsy moth pheromones (+)- and (-)-disparlure [287]. The synthesis required the production of the [y-(alkoxy)allyl]stannane reagent 440 (Scheme 11-32). 

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). ... 

Chiral non-racemic y-[alkoxy]allylic stannanes, such as 275, react with simple aldehydes to yield vyn-l,2-diol derivative 276 in high enantiomeric excess. The anti-SE arrangement in 277 minimizes electronic and steric interactions. 

Marshall has developed a collection of chiral a-alkoxy allyl stannanes 152 that provide facile access to various diastereomeric 1,2-diols (Scheme 5.26) [104-106]. For these reagents, Marshall has proposed that stereospecific rearrangement of 152 in the presence of Lewis acids gives the (Z)-y-alkoxy stannanes 153 prior to aldehyde addition. The transition state for the subsequent addition to the aldehyde is formulated as an open acyclic structure, leading to 1,2-syn-configured diols 154 (cf transition state 96) [107, 108]. 

A. y-Oxygen-Substituleel Stannanes. Oxygenated allylic stannanes have been synthesized and used advantageously in several types of syntheses. Both a- and y-alkoxy and silyloxy stannane can be prepared by several complementary methods.177 C-y-Alkoxy and silyloxy allylic stannanes react with aldehydes to give primarily syn... 

These reagents are also useful for the preparation of 1,2-diols. Upon exposure to Lewis acids such as boron trifluoride etherate (BFa-OEta), the a-alkoxy and oc-siloxyallyl stannanes undergo a stereospecific, intermolecular 1,3-isomerization to give y-alkoxy- and y-siloxy allylic stannanes.3. .7 When tert-butyldimethylsilyl trifluoromethanesulfonate is substituted for chloromethyl methyl ether in the above procedure, the isomeric -y-siloxy allylic stannane can be obtained directly with no loss of enantioselectivity.6 These stannanes can then be added to various aldehydes to give monoprotected 1,2-diols with high diastereoselectivity.8... 

Optically active allyl stannanes thus obtained were treated with a-alkoxy aldehydes to give useful substrates for the synthesis of carbohydrates13. 

The presence of an a-alkoxy substituent on the substrate aldehyde leads to the formation of a differentially protected 1,2,3-triol upon addition of a y-oxygenated allylic stannane (Eq. 43). Such additions are of potential use for the synthesis of carbohydrates and extended polyols. The resident double bond in the adduct can be further functionalized by dihydroxylation or epoxidation to extend the polyol chain. 

Chiral allylic stannanes and chiral aldehydes. Pairwise combination of these chiral allylstannane reagents with chiral a- and jS-alkoxy aldehydes revealed a... 

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... 

Use of y-alkoxy-substituted allylic stannane reagents with a-benzyloxy aldehydes provides convenient access to 1,2,3-triol subunits (Equation 8) [92]. Keck reported the chelation-controlled formation of 113 as a single dia-stereomer and suggested that this product is the result of the intermediacy of transition state structure 112, analogous to 100. 

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... 

Side-chain halogenated amino acid derivatives are reduced, deuterated, allylated and alkylated with stannanes and related reagents in free-radical processes [53]. Other side-chain functional groups may also be manipulated to produce amino acid radicals [8, 54-58], and, in particular. Barton decarboxylation of aspartate and glutamate derivatives has been applied in this manner (Scheme 8) [55]. Related procedures have been developed to generate amino acid radicals by dehydrox-ylation of hydroxy amino acid derivatives [56]. Hydroxy amino acid derivatives may also be converted to nitrate esters, from which the corresponding alkoxy radi-... 

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