Selenoaldehydes synthesis

This synthesis was accomplished using selenoaldehyde 5R. Again, cyclocondensation with 6 gave ent 7. Using the same steps as were used for the NeuSAc synthesis, ent 7 was converted to ent 13. From there the steps to the naturally occurring (7R) antipode of KDO merely involved retracing steps followed in the synthesis of the racemate. 

Selenoaldehydes.1 The first step in a synthesis of selenoaldehydes involves addition of dimethylphenylsilyllithium2 to aldehydes followed by trapping of the adduct with TsCl. The a-silyl tosylates are converted into a-silyl selenocyanates by reaction with KSeCN catalyzed by 18-crown-6. These products are easily purified, and are converted by fluoride ion into selenoaldehydes, which can be trapped by Diels-Alder cycloadditions (equation I). 

Selenoketones.2 The synthesis of selenoaldehydes via selenocyanates (13,286) has been extended to a synthesis of selenoketones. Electron-deficient selenoketones are simply prepared by reaction of halides with KSeCN followed by base-induced elimination of HCN (equation I). However, this route fails in the case of precursors... 

The synthesis of metal complexes of type 213 can be performed by reacting metal-carbene complexes with selenium sources such as alkyneselenolates 203.430 Also the stability of unstable selenocarbonyl compounds such as selenoaldehydes can be enhanced by coordination to metal carbonyls and the reactivity of such complexes has been studied. Complex 216 can react with methylthiohexyne and the product is a different complex 217 with the selenium atom still coordinating to the metal carbonyl fragment (Scheme 66).431... 

Studies of hetero Diels-Alder condensation that used enantiopure substituted aldehydes found that the catalyzed additions had a high and predictable stereoselectivity [231]. The synthesis of iV-acetylneuraminic acid 130 [232] from diene 127 and (S)-selenoaldehyde 128, which is shown below, illustrates this point (O Scheme 44). 

Recently, the synthesis of selenoaldehyde was examined by the reaction of O-alkyl selenoformates (85) with aryllithium. " 2,4,6-Tri-f-butylphenyllithium reacted with selenoformates (85) at three different sites, i.e. the selenoformyl carbon, the selenoformyl hydrogen and the selenium, to give several reaction products including 6,8-di-r-butyl-3,4-dihydro-4,4-dimethyl-l//-2-benzoselenin (86). The formation of (86) is explained in terms of the intermediacy of 2,4,6-tri-r-butylselenobenzaldehyde (87 Scheme 23). 

In this section, the asymmetric synthesis of the vicinal thio- and selenoalcohols 42 — 45 is described based on the highly enantio- and diastereoselective addition of diethylzinc reagent to racemic a-thio- and selenoaldehydes 41, catalyzed by 20o (( —)-DFPE) and S,R)-2Qo ((+)-DFPE) (Scheme 3-20). Although the enantio-selective addition of dialkylzinc reagents to achiral aldehydes using chiral catalysts has been well investigated [10], there are no known catalytic enantio- and di-asteroselective dialkylzinc additions to aldehydes with chiral centers, except for the alkylation of a-methyl- [58, 59], a-chloro- [59], and j5-alkoxyaldehydes [60]. The reaction of diethylzinc with racemic a-thio- and selenoaldehydes 41 was carried out in the presence of 20o or (S,il)-20o (5 — 50 mol%) in hexane at room temperature for 12 —16h. The results are summarized in Table 3-11. 

Synthesis.—No report concerned with the preparation and identification of stable thioaldehydes has appeared however, pyrrolo(2,l- >lthiazole-7- (1) and -5-carbo-selenoaldehydes (2)—(4) have been synthesized by the selenoformylation of pyrrolo[2,l-6]thiazoles and characterized by their H n.m.r. and u.v. spectra. With the exception of (4), these selenoaldehydes are stable, green, crystalline compounds. 

Addition of PhMc2SiLi to aldehydes and subsequent 0-sulfo-nylation of the generated alkoxides with /7-toluenesulfonyl chloride affords a-silyl tosylates in good yields. Conversion of these intermediates into a-silyl selenocyanates followed by fluoride-induced cyanide elimination reaction permitted the synthesis and study of reactivity of previously unknown selenoaldehydes (eq A) ... 

---