Stannyl acetals

General Procedure for the Preparation of 1,2-O-Stannyl Acetals from Hemiacetals and the Glycosylation... 

The reaction of 153 with the stannylated acetate 162 (Scheme 36) shows the directing effect of the Bu3Sn group. Both direct allylation and rearrangement of 164 give 163. Tin elimination from 163b (step c) results in the incorporation of a branched 2-butenyl chain in 165 (92ACS1215). 

The stemona alkaloid stemonamide (49) was synthesized starting from a-stannyl acetate 47 and 2-stannyl pyrrolidine 48. The oxidative coupling of stannyl acetate 47 with acetylenic silyl enol ether affords the functionalized C-7 unit which corresponds to the side arm of the pyrrolidine ring. Then, introduction of the C-7 unit to the pyrrolidine ring is performed by the oxidative generation of acyliminium ion. The carbon skeleton of stemonamide was thus constructed efficiently as shown in Scheme 19 by employing organotin compounds. ... 

The classical access, developed by Still, requires the resolution of a racemic 1 -tri-alkylstarmyl-1 -alkanol [2,3,10]. A great improvement was made by asymmetric reduction of acylstannanes 15, which provides the starmyl alcohols 16 with approx. 80 to 96% ee [Eq. (5)] [ 11 ]. An alternative access, developed by Nakai, uses the nucleophilic ring opening of chiral a-stannyl acetals [12]. Another highly enantioselective approach starts from chiral (l-chloroalkyl)boronates [13]. 

The trichloromethy1-3-bromoalkanes (38) also add to carbonyl groups in a formal [3+2]-type cycloaddition reaction under electrolytic conditions where the intermediate is thought to be the dichloromethyl anion (39)." Trost has extended the scope of his [3+2] eyeloaddltlon reaction to include the synthesis of tetra-hydrofurans. He found that the stannyl acetate (40) rather than the corresponding silyl acetate gives high yields of the... 

Penta-coordinated stannates undergo the Michael reaction with enones and nitroalkenes or cyanoalkenes due to the high nucleophilicity (Schemes 3-201, 3-202, and 3-203). This strategy is applicable to ketene stannyl acetals, which are less reactive than ketene silyl acetals due possibly to the easy equilibrium between 0-Sn and C-Sn tautomers, the C-Sn tautomers being predominating at temperatures above 0 Normally, Qf-stannyl esters are much less reactive with electrophiles. However,... 

Pd(Ph3P)4 and Bu3SnH convert the Alloc group to other amine derivatives when electrophiles such as (B0C)20, AcCl, TsCl, or succinic anhydride are added. Hydrolysis of the stannyl carbamate with acetic acid gives the free amine. 

Fully acetylated hexopyranoses react with thiols in the presence of Lewis acids, such as BF3-Et20.32,33 The reaction is faster with 1,2-trans acetates than with the corresponding 1,2-cij ones and 1,2-trans products predominate. Alkyl, alkenyl, and aryl thioglycosides are produced by this method. Variations on this method include the use of trimethylsilyl34 or stannyl derivatives35 of the thiols. 

Previous syntheses An example of this point can be recognized by examination of one known synthesis of thienobenzazepines (Scheme 6.1). This synthetic route involves a key palladinm-catalyzed cross-conpling of stannyl intermediate 3, prepared by method of Gronowitz et al., with 2-nitrobenzyl bromide. Acetal deprotection and reductive cyclization afforded the desired thienobenzazepine tricycle 4. In support of structure activity relationship studies, this intermediate was conveniently acylated with varions acyl chlorides to yield several biologically active componnds of structure type 5. While this synthetic approach does access intermediate 4 in relatively few synthetic transformations for stractnre activity relationship studies, this route is seemingly nnattractive for preparative scale requiring stoichiometric amounts of potentially toxic metals that are generally difficult to remove and present costly purification problems at the end of the synthesis. 

Arylacetate esters have been generated by coupling aryl bromides with stannyl enolates generated from silyl ketene acetals. 

Telluro(seleno)ketene acetals, 1-seleno-2-telluro-ethenes, telluro ketene acetals, telluro(stannyl)ketene acetals and telluro(thio) ketene acetals... 

Following the precedent methodologies, telluro(stannyl) ketene acetals are achieved by the hydrozirconation of stannyl acetylenes and successive reactions with butyl tellurenyl... 

Kiigelrohr apparatus. Flash column chromatography (hexane) of the residue afforded ketene stannyl(telluro) acetals as yellow liquids. 

General procedure for the synthesis of a-bromovinyl tellurides from ketene stannyl (telluro) acetals To a solution of the appropriate ketene stannyl(telluro) acetal (1.0 mmol) in CH2CI2 (10 mL) cooled at -78°C (at 0°C for 5 h) was added dropwise a solution of NBS (0.445 g, 2.5 mmol) in CH2CI2 (25 mL). Th reaction mixture was stirred for 6 and 12 h (R=H). It was transferred to an Erlenmeyer flask and treated with a solution... 

TABLE 7. Thia-[2,3]-Wittig rearrangements of stannylated precursors and i, i -acetals ... 

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