Epoxy acetals

Hydrolysis of the intermediate epoxy acetate is generally carried out with strong alkali if base-sensitive groups are present, milder conditions (e.g., potassium bicarbonate, potassium carbonate) can be employed. If commercial peracetic acid (which contains sulfuric acid) is used for epoxidation, the intermediate epoxy acetate cannot be isolated, but is hydrolyzed in situ by the acid to the desired ketol. Acid hydrolysis will also retain 3-acetates, if present. ... 

As already mentioned, epoxidation of a A -20-acetoxypregnane always produces the 17a-epoxide. If the epoxy acetate is allowed to stand on a silica gel column, or is heated, it rearranges to the 17l5-acetoxy-17a-pregnan-20-one ° this may be pictured as follows ... 

N3 - —NH2. Some time ago Staudinger2 reported this transformation by reaction of an azide with P(QH5)3 to form an iminophosphorane, RN=P(C6H5)3, which is hydrolyzed to RNH2 and (C6H5)3P=0. This reaction has received little attention, but recent work indicates that it is generally useful, and is compatible with epoxy, acetal, nitro, and ester groups. Yields are 80-95% (seven examples).3... 

For epoxy acetals, a complete reverse in regioselectivity is attained with the zinc/trimethylchlorosilane system as compared with LAH (equation 10). ... 

Unsaturated aldehyde acetals may be transformed either into unsaturated esters or into epoxy acetals. Peroxyacetic acid in ethyl acetate epox-... 

Scheme 9. Dimerization of the cholestanyl amino oxime 16 and epoxy acetate 17.

The utility of the amino oxime ether - acetoxy ketone combination was shown using two different steroid monomers. Androstanolone was converted to the 2P, 17 3-diacetate 24 (Scheme 11), which was then reacted with amino oxime ether 22 to afford unsymmetrical dimer 25 in reasonable yield. In this dimerization, replacement of 24 by its 2a-acetoxy epimer gave similar results, while the 2a-bromoketone instead led to messy reaction mixtures. Reactions between 22 and 2,3-epoxy acetates were also tried, which produced an approximately 1 1 ratio of cis and trans pyrazine dimers. 

It is interesting to note that on phosphorus oxychloride treatment the acetoxy diol (153) is transformed into the epoxy acetate (152), but the dehydration products were not formed (Scheme 25). 

Epoxidation of the A -enol acetate (583) gave the -epoxide (584). Pyrolysis of the epoxy-acetate afforded the 4a-acetoxy-5 -3-ketone (585), which was epimer-ized in acetic acid sodium acetate to give the stable 4)3-isomer (586).There... 

A solution of epoxy acetate 54 (2.07 g, 7.39 mmol, 1.2 equiv.) in 2-methyl-A-pyrrolidinone (17 mL) was added via cannula to a reaction vessel containing Pd2(dba)3 (677 mg, 0.739 mmol, 0.12 equiv) and flame dried LiCl (1.25 g, 29.6 mmol, 4.7 equiv.) at 25 °C. A 3 mL wash of 2-methyl-A-pyrrolidinone was used to quantitate the transfer. A solution of stannane 53 (2.90 g, 6.31 mmol, 1.0 equiv) and i-Pr2NEt (2.58 mL, 14.8 mmol, 2.3 equiv.) in 2-methyl-A-pyrrolidinone (17 mL) was then added via cannula, again using a 3 mL wash of 2-methyl-A-pyrrolidinone to quantitate the transfer. The resultant dark red solution was stirred at 25 °C for 10 min, and then was warmed to 35 °C and stirred for an additional 1.5 h. Upon completion, the reaction contents were cooled to 25 °C, poured into saturated... 

In the case of epoxy acetate 1, a neighboring group participation mechanism should proceed by initial protonation of the epoxide, followed by backside attack of the acetoxy group, ring opening and subsequent formation of acetoxonium ion 5 (Seheme 25.3). 

Carley, S. and Brimble, M.A. (2009) A novel approach to the CDE ring system of pectenotoxins-4 triggered by VO(acac)2-induced epoxy-acetalization. Org, Lett, 11, 563-566. 

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