Allenylstannanes chiral

Chiral allenylstannanes can be prepared by Sjv2 displacement of propargylic halides sulfinates or sulfonates with tin cuprates (Table 14)78. The nonracemic propargylic mesylate (74) afforded a nonracemic allene, [a]D —570, whose configuration was assigned by application of Brewster s rules (equation 38)78. Displacements on the steroidal mesylates 75 and 76 afforded the allenic products with complete inversion of configuration (Scheme 32)78. 

Transmetallation of allenylstannane with InCl3 and subsequent addition to a chiral aldehyde leads to the anti,anti and anti,syn adducts (Equation (17)).171,172... 

Allenylstannanes of 90 % ee are obtained from the corresponding chiral propargylic mesylates 15 (available in ca. 90% ee) in the reaction with the reagent prepared from tributylstannyllithi-um and dimethylsulfide19. 

Throughout this section, the P and M nomenclature (Scheme 5.2.63) will be used to describe absolute configurations of chiral allenylstannanes. 

Transmetalations of allenylstannanes represent an important conceptual development, which substantially increases the potential utility of these reagents. i 4 Corey established an early example via the transmetalation of 3-triphenylstannyl-l-propyne with (/ ,f )-B-bromo-4,5-diphenyl-l,3-diaza-2-borolidine (R,R-22S) yielding the intermediate formation of allenylborane 334. Low temperature condensations of 334 with aldehydes occurred via closed transition states in which the chirality of the Stein auxiliary determined the stereogenicity of the enantioenriched -alcohol 335 (Scheme 5.2.70). ... 

The ramifications of Se transmetalations for chiral allenylstannanes are briefly summarized in Scheme 5.2.72 in which the common electrophile, (2f )-3-benzyloxy-2-methylpropanal, serves to selectively generate two distinct diastereoisomers 336 and 337. The reaction with (M)-286 and SnCU proceeds via anti-SE transmetalation followed by stereospecific isomerization to give the (P)-trichloroallenylstannane... 

In this case, the reversible coordination of tin with aldehyde favors the closed transition state for Felkin-Anh addition in 339, whereas the jS-chelation model of 340 introduces destabilizing steric interactions owing to placement of the methyl substituent of the chiral allene. Transmetalations of chiral allenylstannanes with InBrs occur with net retention of allene geometry (Scheme 5.2.64). Thus, the starting (P)-286 can also be utilized for a stereoselective reaction with the corresponding (5 )-aldehyde (Scheme 5.2.72, bottom). The enantiomeric alcohol ent-337 is produced via the closed transition state... 

In a related example, Hegedus has incorporated chirality into the functionalization of the allenylstannane. Lewis acid-promoted additions of stannylallenamide 359 to simple aldehydes and imines are high i yn-selective processes (Scheme 5.2.76)." The reaction is presumed to occur via antiperiplanar 362, illustrating the anti-SE characteristics of stannyl substitution with a minimization of non-bonding interactions. 

Scheme 5.2.77 Diastereoselection in reactions of nonracemic allenylstannanes leading to chiral allenylal-cohols...

The additions of chiral nonracemic allenylmetal reagents to chiral a-methyl propanal derivatives have been proven useful for the assembly of polypropionate fragments. These reagents rely on allene chirality to favor one of the two possible diastereomeric transition states in the addition and, thus, differ in a fundamental way from the aforementioned methods in which a chiral auxiliary or catalyst provides the control element. For example, a chiral allenylstannane 246 is added to a chiral aldehyde (S)-230, derived from the Roche ester, in the presence of various Lewis acid promoters to afford any of the four diastereo-mers with excellent diastereo- and enantioselectivity, depending on the reaction conditions. Representative results are depicted in Scheme 10.48. From the stereocontrol point of view, these transformations follow Cram-fike open transition state models without or with chelation, respectively. If InBr3, SnCLi, BuaSnCl, or other additives... 

Marshall JA, Wang X. Synthesis of enantioenriched homo-propargylic alcohols through diastereoselective SE additions of chiral allenylstannanes to aldehydes. J. Org. Chem 1992 57 1242 1252. 

Use of these chiral allenyl metal reagents for the introduction of a propar-gyl group allows the synthesis of all four diastereomeric permutations of dipropionate subunits. The propargyl unit also provides a convenient handle for further elaboration, as demonstrated in Marshall s synthesis of the cytotoxic polyketide discodermolide (182, Scheme 5.30) [115]. Propargylation of aldehyde 177 with an allenylzinc species derived from mesylate 172 furnishes anti product 178 in 90 10 dr. The addition of allenylstannane 180 to aldehyde 179 affords the syn product 181 in an impressive 97% yield and >95 5 dr. 

---