Dialkyl tartrates, chiral reagents

Asymmetric induction in the cyclopropanations of unsaturated substrates with methylene has been extensively investigated. A propensity of the Simmons-Simth and related reagents to make coordination to basic atoms is most frequently exploited. Treatment of a,/J-unsaturated aldehyde acetals derived from the aldehydes and chiral dialkyl tartrates or 2,4-pentanediol, with diiodomethane/diethylzinc in hexane, produces cyclopro-panecarboxaldehyde acetals with high diastereoselectivity (equation 69)109 110. Uniformly good diastereoselectivity has also been realized in the cyclopropanations of chiral acetals... 

The known allylic alcohol 9 derived from protected dimethyl tartrate is exposed to Sharpless asymmetric epoxidation conditions with (-)-diethyl D-tartrate. The reaction yields exclusively the anti epoxide 10 in 77 % yield. In contrast to the above mentioned epoxidation of the ribose derived allylic alcohol, in this case epoxidation of 9 with MCPBA at 0 °C resulted in a 65 35 mixture of syn/anti diastereomers. The Sharpless epoxidation of primary and secondary allylic alcohols discovered in 1980 is a powerful reagent-controlled reaction.12 The use of titanium(IV) tetraisopropoxide as catalyst, tert-butylhydro-peroxide as oxidant, and an enantiopure dialkyl tartrate as chiral auxiliary accomplishes the epoxidation of allylic alcohols with excellent stereoselectivity. If the reaction is kept absolutely dry, catalytic amounts of the dialkyl tartrate(titanium)(IV) complex are sufficient. 

Asymmetric allylation is performed by chiral allylating reagents. The chiral allyltin generated in situ from benzodio-stannole [Sn(02C6H4)], allyl halide, chiral dialkyl tartrates and l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) reacts with aromatic aldehydes in the presence of copper salts to afford the chiral homoallyl alcohols (Equation (50)).164... 

Monoallylstannanes, AllylSnX[N(SiMe3)]2, are formed when the stannylene, Sn[N(SiMe3)]2, reacts with allyl bromide,15 17 and the reaction of allyl bromide with the tin(II) catecholate complex of a dialkyl tartrate,18 or with the oxathiastannolane complex of (R)-binaphthol19 gives chiral allyltin reagents. 

Easily accessible acetals and ketals of a,p-unsaturated aldehydes and ketones derived fi-om C2-symmetric chiral 1,2-diols have been successfully used with Simmons-Smith reagents furnishing cyclopropane aldehydes with high selectivity and recovery of the auxiliary. Thus, dialkyl tartrates proved to be superior compared to 1,2-diphenyl-ethanediols as chiral auxiliaries in reactions of a,p-unsaturated aldehydes. 

Catalytic properties of external chiral additives such as (2S,3/ )-4-dimethyl-amino-l,2-diphenyl-3- methyl-2-butoxide (A 16) (574, 575) and 2-magnesium-3-zinc salts of dialkyl (f ,f )-tartrate (A17) were employed in the highly stereoselective addition of organozinc reagents to derivatives of 3,4-dihydro-isoquinoline-A-oxide (Scheme 2.147) (576). 

Seebach and Daum (75) investigated the properties of a chiral acyclic diol, 1,4-bis(dimethylamino)-(2S,35)- and (2K,3/ )-butane-2,3-diol (52) as a chiral auxiliary reagent for complexing with LAH. The diol is readily available from diethyl tartrate by conversion to the dimethylamide and reduction with LAH. The diol 52 could be converted to a 1 1 complex (53) with LAH (eq. [18]), which was used for the reduction of aldehydes and ketones in optical yields up to 75%. Since both enantiomers of 53 are available, dextro- or levorotatory products may be prepared. The chiral diol is readily recoverable without loss of optical activity. The (- )-52-LAH complex reduced dialkyl and aryl alkyl ketones to products enriched in the (S)-carbinol, whereas (+ )-52-LAH gives the opposite result. The highest optical yield of 75% was obtained in the reduction of 2,4,6-... 

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