Allyl additions allylstannane reagents

The a-bromo-7-lactone 901 undergoes smooth coupling with the acetonyltin reagent 902 to afford the o-acetonyl-7-butyrolactone 903[763j. The o-chloro ether 904, which has no possibility of //-elimination after oxidative addition, reacts with vinylstannane to give the allyl ether 905, The o -bromo ether 906 is also used for the intramolecular alkyne insertion and transmetallation with allylstannane to give 907[764],... 

Although these allylic stannanes are rather resistant to uncatalysed or Lewis acid-catalysed carbonyl addition , they are valuable, shelf-stable homoenolate reagents (see Section IV.C.5), which are activated by Lewis acids or lithiodestannylation. Titanium tetrachloride converts the allylstannanes stereospecifically with inversion into very reactive intermediates (equation 83) . Both isomers, (R,Z)- and (5, )-315, are transformed... 

In addition to allylsilanes, CM can also be applied to allylstannanes, which serve as valuable reagents for nucleophilic additions and radical reactions.To date, only eatalyst 1 has been shown to demonstrate CM reactivity in the preparation of 1,2-disubstituted allylstannanes, as ruthenium catalysts were found to be inactive in the presence of this substrate class.Poor stereoselectivities were generally observed, with the exeeption of one instance of >20 1 Z-selectivity in the reaction of allyltributylstannane with an acetyl-protected allyl gluco-side. 

Substitution at the terminal position of the allylstannane, as in crotonyltributyl stannane, however, is not tolerated, because hydrogen abstraction from the allylic position is a competing reaction [21], An extension of the method involves the coupling of the anomeric radical precursors 28 with the allyltributyltin reagent 29 [14], In the reagent 29 the double bond is activated toward addition of nucleophilic radicals by the electron-withdrawing t-butoxy carbonyl group. The obtained product 30 has been useful en route to 3-deoxy-D-marmo-2-octulosonic acid (KDO). 

The Lewis acid catalyzed conjugate addition of allylsilanes (140) to (142) and allylstannanes (154) and (155) to ot,0-enones, described by Sakurai,68a,68b is highly efficient and experimentally simple in contrast to the allylcuprate additions. Various substituents can be incorporated into the allylsilanes (allylstannanes), e.g. alkoxy, alkoxycarbonyl and halogen, some of which are incompatible with cuprate reagents 69 In addition, Heathcock and Yamamoto report that diastereoselectivity is correlated to the alkene geometry of both the allylmetals and the acceptor units for example, allylation of ( )-enones (143) and (146) affords predominantly the syn adducts (144) and (147), while (Z)-enone (149) gives predominantly the anti adduct (150 Scheme 25).680 On the other hand, with cyclohexen-2-one the (Z)-silane (141) affords predominantly the threo adduct (152), while (142) affords erythro adduct (ISS).686 The more reactive allylstannanes (154) and (155) also afford similar diastereoselectivity.68e,f... 

At present, bimolecular allylation is limited to unsubstituted and 2-substituted allylstannanes (which are symmetrical so that addition and elimination of a tin radical are degenerate). 1-Substituted allylstannanes (for example, crotylstannane) are not useful reagents because the substituent decelerates the rate of the radical addition step below the useful limit.139 The usefulness of 3-substituted allylstannanes has been limited by their relatively facile isomerization to the inert 1-substituted isomers under typical reaction conditions.140... 

Carbonyl Allylation and Propargylation. Boron complex (8), derived from the bis(tosylamide) compound (3), transmeta-lates allylstannanes to form allylboranes (eq 12). The allylboranes can be combined without isolation with aldehydes at —78°C to afford homoallylic alcohols with high enantioselectivity (eq 13). On the basis of a single reported example, reagent control might be expected to overcome substrate control in additions to aldehydes containing an adjacent asymmetric center. The sulfonamide can be recovered by precipitation with diethyl ether during aqueous workup. Ease of preparation and recovery of the chiral controller makes this method one of the more useful available for allylation reactions. 

The transmetallation or the metal-metal exchange reaction of an allylic tin species with an electrophile was first observed in 1970 [77]. The possibility that transmetallation may play a role in the Lewis acid-promoted reaction of allylstannanes with aldehydes was initially discussed by Tagliavini [78], Keck [79], Yamamoto [71b, 80], and Maruyama [81]. It is believed that upon transmetallation with either SnCU or TiCl4, the addition of an allylstannane and aldehyde will occur via a cyclic, six-membered transition structure. The reaction occurs by coordination of the aldehyde carbonyl with the Lewis acidic trichlorotin or trichlorotitanium reagent, thus affording the anti homoallylic alcohol (Scheme 10-42). 

Since the discovery of thermally promoted allylation of aldehydes [9], allylstannanes have been widely used in organic synthesis as stable and stereodefined reagents for C-C bond formation. Although it had been reported that activated aldehydes [10] or allylstannanes with chloride on the tin [11] could be used for allylation, remarkably innovative technology for allylation was advanced by Naruta and by Sakurai and Hosomi [12]. They disclosed that allylation was promoted by addition of a Lewis acid this substantially expanded the versatihty of the aUylstaimane procedure. Because many allylation reactions have already been documented [1], the most recent progress in this field will be described after brief description of fundamental aspects. 

Activation by TMSCl has also been applied to the allylstannane allylation of aldimines ", the copper-catalysed conjugate addition of organoaluminium reagents to enones, and in the uncatalysed conjugate addition of organozincs in THF/iV-methylp3Trofidone. Its involvement in the conjugate addition of stabilized... 

Asymmetric induction of chirality is also observed for the diastereoselective addition of organoaluminum reagents and allylstannanes to aldehyde groups at the side chain of Ti -allyl(tricarbonyl)iron lactone complexes. Ketones in the side chain of Ti -allyl(tricarbonyl)iron lactone complexes can be transformed into tertiary alcohols in a diastereoselective fashion by addition of organoaluminum reagents (Scheme... 

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