Ozonolysis, stereospecific

A stereospecific total synthesis of polyoxin C and related nucleosides is reported, in which thereacdon of l-Cphenylthioi-l-nitroalkenes v/ith nucleophiles and siibseqiientozono lysis are key reacdons Addidonof potassium trimethylsilanoate to l-Cphenylthioi-nitroalkenes derived from D-ribose followed by ozonolysis gives the cr-hydroxy thioester, which is formed v/ith excellent diastereoselecdvity fScheme 4 5 This conformadon meets the stereo-electronic requirements for andperiplanar addition of the nucleophile with the result of high 5-fS stereochemical bias in the reacdons... 

Much effort this year has been expended on chrysanthemic acid syntheses. Aratani et al. have extended earlier work on asymmetric synthesis (Vol. 6, p. 21) by decomposing various alkyl diazoacetates in 2,5-dimethylhexa-2,4-diene in the presence of chiral copper complexes to yield up to 92% of rrans-chrysanthemic acid in 88% dextrorotatory enantiomeric excess. Mitra has used ozonolysis of (+)-a-pinene to obtain, stereospecifically, the bromo-ketone (104) which undergoes Favorskii rearrangement to yield the anticipated ester (105) from which (+)-trans-chrysanthemic acid is readily obtained a second paper reports another route from (+)-car-3-ene initially to methyl (—)-c/s-chrysanthemate or to (—)-dihydro-chrysanthemolactone (106), both of which are convertible into (+)-rra s-chrysan-... 

An interesting problem in stereoisomerism is found in the aldol reactions of the achiral aldehydes which are obtained by ozonolysis of the homoallylic alcohols 174. After stereospecific conversion by the FruA [230], the products can be readily induced to form an intramolecular glycoside 175 by acidic (R=OH) or alkaline treatment (R=C1), under which conditions the two equatorial ring hydroxyl groups completely direct the stereogenic acetal formation [234]. The corresponding RhuA catalyzed reactions deliver the enantiomeric... 

The contribution of Ic is not considered likely the possible rotation about the carbon-carbon <7 bond would result in a non-stereospecific ozone addition on the olefin—in violation of Greenwood s observations (21,22) that a trans- (or cis-) olefin yields only trans- (or cis-) primary ozonide. Retention of configuration is also shown in those cases in which epoxide is formed (20). Finally, a recent communication by Bailey et al. (23) reports the formation of free radicals, arising from the homolytic dissociation of a w or a complex, in the ozonolysis of terminal olefins with bulky substituents capable of stabilizing free radicals. 

The relative stereochemistry of the substituents attached to the cyclopropane ring of presqualene alcohol received further confirmation by a synthesis of the triacetate (13), obtainable from (1) by ozonolysis, reduction, and acetylation. Treatment of 3-methyl-truns-hex-2-ene-l,6-diacetate (14) with ethyl diazoacetate in the presence of copper powder gave two isomers (15) and (16) whose stereochemistry was assigned by n.m.r. The triacetate derived from synthetic (15) by reduction and acetylation was identical in all respects with the triacetate (13) from natural presqualene alcohol. Presqualene alcohol has a c.d. curve similar to, but of opposite sign to, that of (li ,2i )-trans-chrysanthemyl alcohol (17). The mechanism for the stereospecific biosynthesis of squalene from farnesol via presqualene alcohol has received detailed comment. ... 

Synthesis of (+)-desoxoprosopinine [(+)-4] through a key amidoalkylation reaction, which permits stereospecific generation of protected tran5-2,6-disubstituted piperidines, has been achieved (Scheme 6) AUylation of 60, obtained from compound 59, occurred rapidly upon treatment with allyltrimethylsilane in the presence of TiCU to provide 61 in 88% yield, which underwent deacetylation and benzylation to furnish 62. Ozonolysis of the allyl group in 62 followed by olefination of the resnlting aldehyde and subsequent... 

Stereospecific osmylation produces lactone 840 as a single diastereomer. This, upon treatment with sodium bicarbonate, gives bicyclic lactone 841 in high yield. Reduction of lactone 841 to a lactol followed by Wittig olefination affords allyltetrahydrofuran 842. Pal-ladium(II) isomerization of the olefin, epoxidation, ozonolysis, and reduction gives the target compound 844 with 95% optical purity. 

Ozone hydro gen peroxide Carboxylic acids from phenols Stereospecific ozonolysis... 

In the last synthesis from this series,the (5)-conhgured a-hydroxyphosphonate building block 70 was required (Scheme 47.17). In this case, the desired carbonate could be obtained directly from the enzymatic kinetic resolution as it was the unreactive enantiomer. The synthesis involved similar key steps as before, namely, a cross metathesis, followed by a stereospecific intramolecular palladium-catalyzed allylic substitution that furnished the furan ring in 78. In contradistinction to the previous syntheses, the phosphonate group was not removed by ozonolysis, but instead it was employed in the Wittig reaction, to produce fragments 80 and 81 for the synthesis of amphidinolides F and C. 

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