Optically active stannanes

Marshall s optically active stannanes are readily prepared by the addition of tributylstannyllithium to crotonaldehyde (158), oxidation of the ensuing alcohol to the acyl stannane (159), and enantioselective reduction with Noyori s BINAL-H [110] (Scheme 5.27) [107]. Protection of the secondary alcohol furnishes a-alkoxy-substituted allylstannane 160 that partakes in the stereo-specific rearrangement to afford 161 in 80% yield. 

Several other control experiments provided further insight into these transformations (Figure 13.3) [60], When optically active stannane 83 was subjected to lithiation at -78 °C in the presence or absence of TMEDA, followed by quenching with TMSCl, the racemic product ( )-82 was obtained. Lithiation of 83 at -78 °C in the presence of (-)-sparteine followed by addition of TMSCl furnished (S)-82 in 62 % ee. Remarkably, treatment of 83 with sec-BuLi and (-)-sparteine at -78 °C, followed by warming to -25 °C for 2h prior to recooling to -78 °C before addition of TMSCl, led to the formation of enantiomeric (R)-82 in 85 % ee. These results constituted a useful protocol for the generation of enantiomeric products in the presence of... 

Tin/lithium exchange on the a-alkoxy stannanes and subsequent addition of carbon dioxide led to optically active (7-protected a-hydroxy acids 18 with retention of configuration and without any loss of stereochemical information11. 

Stannane 37 lost 50% of its optical activity when allowed to stand as a 0.2 M solution in benzene for 17 days. Addition of A1BN to the solution at 80 °C caused complete racem-ization after 30 min. With added hydroquinone, the benzene solution at 80 °C showed no decrease in rotation after 2 h. It was thus concluded that racemization proceeds by homolysis. 

The transmetallation process was extended to the preparation of vinylboranes709,710, and the superior reactivity of organotins over organosilicons was elegantly demonstrated by Williams and coworkers for the preparation of the optically active allylborane 24 from the corresponding allylic stannane in the total synthesis of (—)-Hennoxazole A711 (equation 54). 

The effectiveness of various substituted BINOL ligands 12-16 in the Zr(IV)-or Ti(IV)-catalyzed enantioselective addition of allyltributyltin to aldehydes was also investigated by Spada and Umani-Ronchi [21], The number of noteworthy examples of asymmetric allylation of carbonyl compounds utilizing optically active catalysts of late transition metal complexes has increased since 1999. Chiral bis(oxazolinyl)phenyl rhodium(III) complex 17, developed by Mo-toyama and Nishiyama, is an air-stable and water-tolerant asymmetric Lewis acid catalyst [23,24]. Condensation of allylic stannanes with aldehydes under the influence of this catalyst results in formation of nonracemic allylated adducts with up to 80% ee (Scheme 3). In the case of the 2-butenyl addition reac-... 

Transformations involving chiral catalysts most efficiently lead to optically active products. The degree of enantioselectivity rather than the efficiency of the catalytic cycle has up to now been in the center of interest. Compared to hydrogenations, catalytic oxidations or C-C bond formations are much more complex processes and still under development. In the case of catalytic additions of dialkyl zinc compounds[l], allylstan-nanes [2], allyl silanes [3], and silyl enolethers [4] to aldehydes, the degree of asymmetric induction is less of a problem than the turnover number and substrate tolerance. Chiral Lewis acids for the enantioselective Mukaiyama reaction have been known for some time [4a - 4c], and recently the binaphthol-titanium complexes 1 [2c - 2e, 2jl and 2 [2b, 2i] have been found to catalyze the addition of allyl stannanes to aldehydes quite efficiently. It has been reported recently that a more active catalyst results upon addition of Me SiSfi-Pr) [2k] or Et2BS( -Pr) [21, 2m] to bi-naphthol-Ti(IV) preparations. 

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

Use of cyclization for access to optically active heterocycies has been studied extensively (Scheme 12.65) [137]. When an enantiomerically defined homoallyloxy stannane was treated with BuLi the desired cychzation occurred to afford the optically active furan derivative and the destannylated compound. Addition of LiCI to this cyclization procedure enhanced the yield to furnish trans and cis products in high enantio excess. 

The effect of solvent was also studied and complexing solvents such as THF or Et20 inhibited the cyclopropanation reaction. Furthermore, the presence of an unprotected allylic alcohol was found to be essential, since the methyl or benzyl ether derived from cinnamyl alcohol afforded almost racemic cyclopropanes. This method has also been extended to the enantioselective cyclopropanation of vinylsilanes and -stannanes (Scheme 4) [13]. The corresponding optically active silyl- and stannyl-substituted cyclopropyhnethanols were obtained in the presence of the chiral N,iV-bis(p-nitrobenzenesulfonyl)-l,2-cyclohexane-diamine 9. 

Asymmetric allylation of aldehydes with allyhc agents catalyzed by Lewis acid is a practical method for synthesizing optically active hranoallylic alcohols [10]. The chloro complex 1 serves as an efficient catalyst for asymmetric allylation of aldehydes with allylstannane [8, 9, 11]. In the presence of 5 mol% of the benzyl-phebox-Rh complex 1-Bn, the coupling reaction of benzaldehyde with allyltributyl-stannane in CH2CI2 at room temperature proceeded smoothly to provide the corresponding homo-aUyl alcohol 7a in 88% yield with 61% ee (Scheme 2). When methaUylstannane was subjected to the reactiOTi, enantioselectivity of the allylated product 8 significantly increased to be over 90% ee. 

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