Reductive cleavage ethers

Preparation of 19-norsteroids from 19-substituted steroids by transformations of 6/3, 19-ethers reductive cleavage of 5o -bromo-6/3,19-ethers, 266... 

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

This group was developed for protection of the 5 -OH group in nucleosides. The derivative is prepared from the corresponding triaiylmethyl chloride, and is cleaved by reductive cleavage (Zn/AcOH) of the phenacyl ether to the p-hydroxyphenyl-diphenylmethyl ether followed by acidic hydrolysis with formic acid. ... 

The reductive cleavage of -bromo ethers is a related reaction. If the ether is cyclic, the reverse reaction takes place on treatment with NBS ... 

Rearrangement of a,/B-epoxy ketones to ftdicarbonyl isomers, 307 Reductive alkylation, 97 Reductive cleavage of halo ethers, 264 Reductive degradation of 19-substitutional steroids, 277, 278 Reformatsky reaction, 139 Removal of the C-10 substituent in steroids. 272... 

A study of the reductive cleavage of a series of alkoxymethyl ethers using the glucose backbone shows that, depending on the reagent, excellent selectivity can be obtained for deprotection vs. methyl ether formation for most of the common protective groups. ... 

The synthesis of the trisubstituted cyclohexane sector 160 commences with the preparation of optically active (/ )-2-cyclohexen-l-ol (199) (see Scheme 49). To accomplish this objective, the decision was made to utilize the powerful catalytic asymmetric reduction process developed by Corey and his colleagues at Harvard.83 Treatment of 2-bromocyclohexenone (196) with BH3 SMe2 in the presence of 5 mol % of oxazaborolidine 197 provides enantiomeri-cally enriched allylic alcohol 198 (99% yield, 96% ee). Reductive cleavage of the C-Br bond in 198 with lithium metal in terf-butyl alcohol and THF then provides optically active (/ )-2-cyclo-hexen-l-ol (199). When the latter substance is treated with wCPBA, a hydroxyl-directed Henbest epoxidation84 takes place to give an epoxy alcohol which can subsequently be protected in the form of a benzyl ether (see 175) under standard conditions. 

The reductive cleavage of allyl ethers 28 30 or sulfides is preferred over the metalation of allyl... 

REDUCTIVE CLEAVAGE OF ALLYLIC ALCOHOLS, ETHERS, OR ACETATES TO OLEFINS 3-METHYLCYCLOHEXENE... 

Reductive cleavage experiments on a series of synthetic -substituted imino ethers have shown that different results would be obtained depending on the nature of the imino system (45). Thus, /3-aryl-substituted imino ethers can be reduced and cleaved with sodium in boiling alcohol whereas /3-aliphatic imino ethers are not susceptible to this kind of reductive fission. The /3-imino ethers synthesized include types 52 and 53. It appears that this kind of reductive cleavage might be developed into a common degradative method for the similar indolenene alkaloids. 

Detailed discussion of these findings will be presented elsewhere. Here we only wish to point out that responses to a hydrogen donor tend to be critically affected by minor structural differences between the compounds. Thus, while diphenyl ether remains substantially unaffected by the donor, its hydroxy-derivatives (phenoxy phenols) often display fairly high reactivity. Taken in conjunction with the failure of low-rank coals (7) and phenoxy phenols (10) to suffer reductive cleavage when treated with sodium in liquid ammonia, this lends some support for the existence of phenoxy phenol entities in low rank coals. 

During attempted reductive cleavage of the ether with aluminium dichloride hydride etherate an explosion occurred. Peroxides may have been present in the susceptible ether. 

The combination of PMHS and Pd(PPh3)4 reduces allyl ethers to propene and alcohols.270 The best combination for the reductive cleavage of ethers appears to be Et3SiH/(C6F5)3B, which gives excellent yields of the alcohol (via the silyl ether) and alkane (Eq. 127).145... 

It was further shown that the nonracemic stannane 70 can be prepared from the Mosher ester derivative 69 through reductive cleavage and BOM protection (equation 28). Lithiation and methylation gave the nonracemic ether with retention of configuration59. 

Si—Siexo av. 2.352/2.409 A) reacts with KCg in diethyl ether by reductive cleavage of an exocyclic Si-Si bond. In the crystal, K(18-crown-6 R 3Si4 shows a separated ion pair and the anion R 3Si4 has a significantly distorted tetrahedran skeleton. 

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