Cleavage base-catalyzed

Keto acids are obtained by acylation of cyclopentanone enamines (see p. I3f.) with acid chlorides and subsequent base-catalyzed mro-aldol cleavage (S. Hdnig, 1960). 

Cleavage reactions of carbohydrates also occur on treatment with aqueous base for prolonged periods as a consequence of base catalyzed retro aldol reactions As pointed out m Section 18 9 aldol addition is a reversible process and (3 hydroxy carbonyl com pounds can be cleaved to an enolate and either an aldehyde or a ketone... 

An asymmetric synthesis of estrone begins with an asymmetric Michael addition of lithium enolate (178) to the scalemic sulfoxide (179). Direct treatment of the cmde Michael adduct with y /i7-chloroperbenzoic acid to oxidize the sulfoxide to a sulfone, followed by reductive removal of the bromine affords (180, X = a and PH R = H) in over 90% yield. Similarly to the conversion of (175) to (176), base-catalyzed epimerization of (180) produces an 85% isolated yield of (181, X = /5H R = H). C8 and C14 of (181) have the same relative and absolute stereochemistry as that of the naturally occurring steroids. Methylation of (181) provides (182). A (CH2)2CuLi-induced reductive cleavage of sulfone (182) followed by stereoselective alkylation of the resultant enolate with an allyl bromide yields (183). Ozonolysis of (183) produces (184) (wherein the aldehydric oxygen is by isopropyUdene) in 68% yield. Compound (184) is the optically active form of Ziegler s intermediate (176), and is converted to (+)-estrone in 6.3% overall yield and >95% enantiomeric excess (200). 

This group is more labile to hydrolysis than the TBDMS group and has been used to protect in alcohol where the TBDMS group was too resistant to cleavage. The DEIPS group is approximately 90 times more stable than the TMS group to acid hydrolysis and 600 times more stable than the TMS group to base-catalyzed solvolysis. 

E. C. Taylor and his co-workers have demonstrated an important principle in the ring-opening of pyridopyrimidines and other fused pyrimidine systems to o-aminonitriles. They have demonstrated that based-catalyzed cleavage of a 4-substituted pyrimidine will occur provided that (a) the anion formed by the attack by the base at the 2-position can be stabilized by appropriate structural features in the remainder of the molecule and (b) that the substituent attached to the 4-position is capable of departure with its bonding pair of electrons in... 

Ring cleavage of l-oxo-l,2,3,4-tetrahydro-jS-carboline derivatives (374) may be accomplished by base-catalyzed hydrolysis to yield tryptamine-2-carboxylic acids (375). In the case of the 1,9-dimethyl derivative decarboxylation accompanied acid-catalyzed ring-opening, and the corresponding tryptamine (376) was obtained directly. 

Poly(L-malate) decomposes spontaneously to L-ma-late by ester hydrolysis [2,4,5]. Hydrolytic degradation of the polymer sodium salt at pH 7.0 and 37°C results in a random cleavage of the polymer, the molecular mass decreasing by 50% after a period of 10 h [2]. The rate of hydrolysis is accelerated in acidic and alkaline solutions. This was first noted by changes in the activity of the polymer to inhibit DNA polymerase a of P. polycephalum [4]. The explanation of this phenomenon was that the degradation was slowest between pH 5-9 (Fig. 2) as would be expected if it were acid/base-catalyzed. In choosing a buffer, one should be aware of specific buffer catalysis. We found that the polymer was more stable in phosphate buffer than in Tris/HCl-buffer. 

When it does occur, the Bal2 mechanism is easy to detect, since it is the only one of the base-catalyzed mechanisms that requires inversion at R. However, in the last example given, the mechanism is evident from the nature of the product, since the ether could have been formed in no other way. The Aal2 mechanism has been reported in the acid cleavage of y-lactones. ° ... 

Conjugated dienes can be acylated by treatment with acyl- or alkylcobalt tetracarbonyls, followed by base-catalyzed cleavage of the resulting jt-allyl carbonyl derivatives. The reaction is very general. With unsymmetrical dienes, the acyl... 

Methyl ketones are degraded to the next lower carboxylic acid by reaction with hypochlorite or hypobromite ions. The initial step in these reactions involves base-catalyzed halogenation. The a-haloketones are more reactive than their precursors, and rapid halogenation to the trihalo compound results. Trihalomethyl ketones are susceptible to alkaline cleavage because of the inductive stabilization provided by the halogen atoms. 

The A-D-ring analog 30a,b (mixture of epimers) has been prepared from the epoxide 3a,b by base catalyzed epoxide cleavage, hydroxymethylenation, and O-alkylation of the butenolide unit using standard conditions. Hydroxymethylenation of keto-ester 7 followed by butenolide addition provided the A-D-ring analog 31. 

Lindsay and Pyne utilized microwave heating for a base-catalyzed cleavage of the oxazolodinone group during the total synthesis of the tricyclic core structure of the cromine alkaloids (Scheme 6.160 b) [263]. 

Several syntheses of l,3-dioxoperhydropyrrolo[l,2-c]imidazoles have been developed using different strategies. a-Substituted bicyclic proline hydantoins were prepared by alkylation of aldimines 135 of resin-bound amino acids with a,tu-dihaloalkanes and intramolecular displacement of the halide to generate cr-substituted prolines 136 and homologs (Scheme 18). After formation of resin-bound ureas 137 by reaction of these sterically hindered secondary amines with isocyanates, base-catalyzed cyclization/cleavage yielded the desired hydantoin products <2005TL3131>. 

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