Epoxide acid-catalyzed cleavage

The dithiane derivative 60 (Scheme 14) is such a compound, it being made from 2,3 5,6-di-0-isopropylidene-D-mannose by treatment with 2-lithio-l,3-dithiane to give a heptose dithioacetal that was refunctionalized at C-2-C-3 by way of the C-l anion and then converted to the 6,7-epoxide following selective acid-catalyzed cleavage of the 6,7-acetal ring. Treated with n-butyllithium it gives, in 70% yield, the cyclized 61, which is efficiently convertible into validatol 62, a component of validamycin A, by desulfurization with Raney nickel and de-O-protection by use of boron tribromide in dichloromethane [31]. 

REACTIONS OF EPOXIDES 1. Acid-catalyzed cleavage. Discussed in Sec. 17.12. 

Compound B also generates compound D when H, H2O is added. This reaction denotes acid-catalyzed cleavage of epoxides. An important characteristic of this reaction is the production of compounds with two functional groups. The general reaction is ... 

Epoxides are highly reactive due to the ease of opening of the highly strained three-membered ring. Epoxides can undergo base and acid-catalyzed cleavage. Reactions in (a) - (c) are examples of such cleavages. 

Acid-catalyzed cleavage of the acetal linkage can also cause initial chain scission. To reduce this acid acceptors are believed to be used in commercial practice. Epoxides, nitrogen-containing compounds, and basic salts are all quoted in the patent literature. 

Scheme 12.15 gives some examples of both acid-catalyzed and nucleophilic ring openings of epoxides. Entries 1 and 2 are cases in which epoxidation and solvolysis are carried out without isolation of the epoxide. Both cases also illustrate the preference for anti stereochemistry. The regioselectivity in Entry 3 is indicative of dominant bond cleavage in the TS. The reaction in Entry 4 was studied in a number of solvents. The product results from net syn addition as a result of phenonium ion participation. The ds-epoxide also gives mainly the syn product, presumably via isomerization to the... 

Cyelobutanone has been prepared by (1) reaction of diazomethane with ketene,4 (2) treatment of methylenecyclobutane with performic acid, followed by cleavage of the resulting glycol with lead tetraacetate,s (3) ozonolysis of methylenecyclobutane, (4) epoxidation of methylene-cyclopropane followed by acid-catalyzed ring expansion,7 and (5) oxidative cleavage of cyclobutane trimethylene thioketal, which in turn is prepared from 2-(co-chloropropyl)-l,3-dithiane.8... 

The base-catalyzed hydration of 2-phenyloxirane involves nucleophilic attack preferentially at C(3) (0-C(3) cleavage), but with only partial regio-selectivity. Acid-catalyzed hydration is mainly by 0-C(2) cleavage. The hydration of 2-phenyloxirane catalyzed by epoxide hydrolase is characterized by its very high regioselectivity for the less-hindered, unsubstituted C(3) [175] [176], involving retention of configuration at C(2). In other words, (R)-and (5)-2-phenyloxirane are metabolized to (/ )- and (S)-l-phenylethane-l,2-diol (10.118), respectively. Substrate enantioselectivity was also character-... 

Step 2 Selective epoxidation of the more electron rich double bond. Step 3a Acid-catalyzed epoxide cleavage. 

Mechanism 8-10 Acid-Catalyzed Opening of Epoxides 362 8-14 Syn Dihydroxylation of Alkenes 364 8-15 Oxidative Cleavage of Alkenes 366 8-16 Polymerization of Alkenes 369 8-17 Olefin Metathesis 373... 

Hydride Reduction of a Carbonyl Group 454 Reaction of a Tertiary Alcohol with HBr(S[ 1) 480 Reaction of a Primary Alcohol with HBr (SN2) 480 Reaction of Alcohols with PBr3 485 (Review) Acid-Catalyzed Dehydration of an Alcohol 487 The Pinacol Rearrangement 495 Cleavage of an Ether by HBr or HI 639 Acid-Catalyzed Opening of Epoxides in Water 649 Acid-Catalyzed Opening of an Epoxide in an Alcohol Solution 650... 

Acid-catalyzed epoxide cleavage takes place by back-aide attack nucleophile on the protonated toxide in a manner analogous to the step of alkene bromtnation, in ndiich a cyclic brentORtunt ion is opened nucleophilic attack (Section 7.2l. When an opoxycycloalkane is opened aqueous acid, a frona-t -diol results just as a traR5 l,2-dibroiiiide from cycloalkene hrominaUun. 

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