Pivaloate ester

In the first step tetrabutylammonium fluoride (TBAF) cleaves the silyl protecting group at C-2a (51). Secondly the pivaloate ester 53 is formed.24 In the last step the ketal was removed under acidic conditions by using trifluoroacetic acid (TFA) in water and CH2C12. 

Pivaloate esters to protect alcohols, 98-99 to protect phenols, 163-164 N-Pivaloyloxymethylamines, to protect amines, 393... 

Alkynyl carboxylate esters, 75, are obtained in the reaction of lithium acetylides with bis(acyloxy)iodobenzene, 73 [Eq. (32)] [59], These reactions are likely to proceed through the intermediacy of the respective alkynyliodonium carboxylates (74), although no such salts have been isolated to date as they spontaneously decompose, via loss of iodobenzene, to the alkynyl carboxylates, 75. Only benzoate esters (75 R = CgHj) are sufficiently stable to isolate and store pure for longer periods. Simple alkylcarboxylates such as acetates are not stable although the hindered pivaloate ester (65 R = t-Bu, R = t-Bu) has been isolated in low yield [59]. Among the reasons for the instability of these esters is their sensitivity to moisture they both readily add water and undergo subsequent hydrolyses [60]. Because of the sensitivity to moisture, the isolated yield [41] of bisalkynyl benzoates, 76, from the bisalkynyliodonium triflates, 35, is only 6-15% [Eq. (33)]. 

With these features in mind the prevalence of acetate, chloroacetate, benzoate (and substituted variants), and pivaloate esters in carbohydrate hydroxyl protection can be readily understood. 

The pivaloate ester of 2-nitro-2-propen-1-ol had previously been demonstrated to be a versatile multiple coupling reagent (Volume 9, Chapter 5, Ref.407, p.3 4). Analogues of 2 -nitroprop-2 -en-... 

Esters (ROCOR ) can be easily prepared by reacting an alcohol (ROH) with an acid chloride (R COCl) and base. Deprotection is generally accomplished by basic hydrolysis or alcoholysis of the ester. Commonly used ester groups include acetate (-COCH3, or Ac) and benzoate (-COPh, or Bz).The pivaloate ester (-COr-Bu, or Pv) is useful for selective acylation of a primary alcohol in the presence of more hindered secondary or tertiary alcohols. 

Hennoxazole A displays potency against Herpes Simplex virus type 1 and peripheral analgesic activity comparable to that of indomethacin. Williams and co-workers reported a total synthesis of (-)-hennoxazole A 141. The Kocienski modification of the Julia-Lythgoe olefination was very successfully employed in the formation of Cn-Cis alkene in 85% yield with excellent iJ-selectivity E/Z = 91 9) by reacting sulfone 140 with aldehyde 139. Hydrolysis of the C4 pivaloate ester (LiOH in aqueous THF/MeOH) provided synthetic hennoxazole A (141) in 72% yield. 

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