Masked formyl group

Dondoni et al. have reported the syntheses of KDN (5) and 4-epi-KDN (29) from protected o-mannose (30a, b) (Scheme 3) [86]. The ylid 31, with the thiazole substituent as a masked formyl group, was added to the protected sugars to give the (E)-a,j0-enones 32a, b in good yield. Conjugate addition of benzyl oxide anion to the enones gave mixtures of syn and anti diastereomers... 

A variation of the Michaelis-Arbuzov reaction uses chloromethyloxazine as a masked formyl group (Scheme 5.6). Chloromethyloxazine, prepared in 55-63% yields by condensation of chloroacetonitrile and 2-methyl-2,4-pentanediol in cold IFSO4, reacts with trialkyl phosphites at reflux to furnish dialkyl oxazinemethylphosphonates in moderate (40%, R = Me) to good yields (80%, R = Et). .io6... 

Unsaturated 3-fonnylphosphonates can be prepared by a Michaelis-Arbuzov reaction that employs l,l-diethoxy-4-bromo-2-butene as the masked formyl group. Reaction with triethyl phosphite at ISS C gives diethyl 4,4-diethoxy-2-butenylphosphonate in 50% yield. Deprotection is accomplished in 98% yield using a cold-saturated aqueous solution of tartaric acid. ... 

The kinetic resolution developed by Katsuki-Sharpless30 for allylic alcohols is superior in enantiotopic face differentiation and in versatility. Another interesting chiral induction method has been developed by Dondoni31 using 2-(trimethylsilyl)thiazole as a masking formyl group. These methods are... 

Sigmatropic rearrangement of phenylthiomethylammonium or sul-phonium ylides forms the basis of a new method for the selective positioning of a masked formyl group (Scheme 88). 

In readily available (see p. 22f.) cyclic imidoesters (e.g. 2-oxazolines) the ot-carbon atom, is metallated by LDA or butyllithium. The heterocycle may be regarded as a masked formyl or carboxyl group (see p. 22f.), and the alkyl substituent represents the carbon chain. The lithium ion is mainly localized on the nitrogen. Suitable chiral oxazolines form chiral chelates with the lithium ion, which are stable at —78°C (A.I. Meyers, 1976 see p. 22f.). 

Substituted malondialdehydes form pyrimidines substituted in the 5-position with an alkyl, aryl, halo, or hetero substituent. The pyrimidine is unsubstituted in the 4- and 6-positions. /3-Dialdehyde equivalents are frequently used in these reactions, for example, 3-alkoxy- or 3-aminoacroleins. With aldehydo ketones, the pyrimidine carries a substituent in the 4- or 6-position. The formyl group in the ketone is normally masked as an alkoxymethylene ketone or as an aminomethylene ketone. A commonly used procedure involves the preparation of a dimethylaminomethyl-ene ketone 645 by reaction of a methyl ketone 644 with DMF dimethylacetal and subsequent reaction with an amidine or guanidine to form the target pyrimidine 646 <2003MI237, 2004JHC461>. 

The indole nucleus in the tryptophan side chain is unreactive enough to be left without protection. Its NH group is not readily acylated but can suffer alkylation to a small but not always negligible extent and some investigators prefer to counteract this side reaction by the application of a masking group. So far only the formyl group found acceptance, perhaps because it is smoothly introduced by the treatment of tryptophan with HCl in formic acid ... 

The Stille coupling reaction is very versatile with respect to the functionality that can be carried in both the halide and the tin reagent. Groups such as ester, nitrile, nitro, cyano, and formyl can be present, which permits applications involving masked functionality. For example, when the coupling reaction is applied to l-alkoxy-2-butenylstannanes, the double-bond shift leads to a vinyl ether that can be hydrolyzed to an aldehyde. 

The oxazolidine system proved a good protecting group with which to mask the ethanolamine moiety in the a-formylation and a-benzoylation of 558, and it could also be used as an aldehyde donor in the rearrangement, based on the ring-chain tautomeric character of 559, under acidic conditions to yield 3-(2-hydroxyethyl)-substituted 1,3-oxazin-4-ones 560 (Scheme 106) <1996JOC3358>. 

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