Crotyl stannanes

Open transition states have been postulated in the aldol-type additions of ( )- and (Z)-crotyl-stannanes (21/22) to aldehydes. Irrespective of the ( ) or (Z) configuration of the stannane only. yyn-adducts 23/24 are formed. Due to the Lewis acid (LA) complexation of the carbonyl oxygen, a cyclic ( closed ) transition state cannot be adopted. Instead, an open geometry is preferred, in which the methyl and the R group move apart as far as possible to generate the enantiomorphous arrangements 25/2611. 

Crotyl stannanes react with good stereochemical control... 

Interestingly, under these conditions, the oxidation appears to be highly ste-reoconvergent and both crotyl stannanes (E)- and (Z)-46 b led to the ( )-allylic alcohol 47 b with an E/Z stereoselectivity better than 95%. This reaction has been extended to other more complex tributylstannyl-l-alkenylcarbamates which were oxidized with a stoichiometric amount of Se02 in the presence of formic acid. Good yields (59-82%) of the corresponding allylic alcohols were obtained. 

Addition of cis- and trans-crotyl tributyltin to benzaldehyde could also be achieved with this catalyst system (Table 21) [37]. Interestingly, the anti product was predominant and the anttsyn ratio was unaffected by the cis.trans composition of the crotyl-stannane. These results are suggestive of an acyclic synclinal transition state for the reaction. 

These findings are consistent with a process in which metathesis of the cw-crotyl stannane is faster than addition of that stannane to the aldehyde at -78 °C (Fig. 8). The resulting secondary BuSnCl2 intermediate adds rapidly to the aldehyde to afford the linear product. At elevated temperature, 1,3-isomerization of the secondary stannane competes with aldehyde addition resulting in more of the branched product. The trans crotyl stannane, on the other hand, is more reactive than the cis isomer and BuSnCls-promoted addition competes with metathesis when the aldehyde is added after short pre-equilibration (1 min). Longer pre-equilibration times enable metathesis and ensuing 1,3-isomerization to occur with the formation of cis- and tranj-crotyl BuSnCla intermediates which react with aldehydes by a cyclic transition state to form the syn and anti branched adducts. 

Table 36. Additions of traws-crotyl and frara-/3-methyl crotyl stannanes to aldehydes. 1 O OH OH R ...

This kinetic resolution has also been effected with a-oxygenated aldehydes (Eq. 47) [67]. Interestingly, in the example shown the mismatched product is the ( )-anti diastereomer rather than the (Z)-syn previously seen with the crotyl stannanes (Eq. 44). Presumably the greater steric requirements of isopropyl compared with methyl disfavors the formation of a (Z) double bond thus rendering the (Z)-syn pathway less favorable in this example. The E)-syn adduct served as a key intermediate in the total synthesis of bengamide E. 

Eqs. (11.48) and (11.49)) [310]. While adduct ent-l06c, obtained with 98 2 diastereo-selectivity, results from the matched crotylation of ent-91c with crotyl-stannane 10, the anti,syn adduct 107c, obtained in 90 10 selectivity, is the result of a mismatched double asymmetric crotylation reaction. However, with these aldehydes, a stoichiometric amount of the CAB catalyst is necessary for synthetic efficiency. 

Nucleophile = Allyl/crotyl silanes, Allyl/crotyl stannanes, Silyl enol ethers, Silyl ketene acetals, Silyl hydrides. Metal cyanides. Alkyl zincs... 

In addition, when crotyltributyltin and Bu2SnCl2 are preheated at 60 °C for 2 h, transmetalation takes place to give dibutylchloro(crotyl)stannane. This subsequently... 

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