De-O-protection

By heating 32 in aqueous dimethyl sulfoxide (DMSO), containing sodium chloride, up to 170°C, loss of a methoxycarbonyl group and then -elimination of acetic acid occur to give alkene 33 in 70% yield. From this, carba-pl-L-mannopyranose 34 has been produced by conversion of the ester group to the hydroxymethyl function, hydroboration, and de-O-protection. Thus, the hydroboration step proceeded by cis-addition anti- to the allylic benzyloxy group. 

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]. 

Hydrazinopyridines reacted with glycosylthioformimidates (e.g., 230) (76JOC3124 79MI1) or 2,5-anhydrohexoaldonic acids (e.g., 79) (78MI12) to yield, after de-O-protection, mixtures of the a- and jS-anomers of 234 (Scheme 69). 

Carbon-carbon bond formation between the aldehydo-2-deoxy-D-hbose derivative 244 and 3-lithio-2-fluoroquinoline produced the two diaste-reoisomers of the quinolin-3-yl acyclo C-nucleoside 245. Separation of the two diastereoisomers, cyclization of their alditolyl chains, and de-O-protection gave the 3-(2-deoxy-/3-D-ribofuranosyl)quinoline 246 and its a-anomer (91TL3297) (Scheme 73). 

When cyclocondensed with 3-hydrazinopyridazine, D-ribofuranosylthio-formimidate (230) gave an anomeric mixture of 295, which was then separated and de-O-protected to the C-nucleoside 296 (81JHC893). Compounds... 

CCC2798) through transformation to the 4-amino-5(3)methoxycarbon-ylpyrazol-3(5)-yl C-nucleoside 411. Cyclization of 411 by heating with formamide and de-O-protection of the sugar moiety produced formycin B (387) (Scheme 115). 

Alkylideneaminopyrimidines added onto the jS-o-ribofuranosyl chloro-ketene 597 to give an adduct that eliminated a hydrogen chloride molecule to furnish, after de-O-protection, the pyrido[l,2-a]pyrimidin-3-yl C-nucleoside 598 (84MI3 85CPB2671) (Scheme 159). 

Under slightly acidic conditions, the pyran ring of the semicarbazone 794 underwent contraction to the 2-formyl-l-ureidopyrrol-5-yl C-nucleo-side 795. The latter was cyclodehydrated to 796 and de-O-protected to 797 [92H(34)569] (Scheme 209). 

Sodium cyanohorohydride-boroB De-O-protection. Both tetrah said to be cleaved with this reagen advantage over existing procedures Reductive dealkoxylation of aci ethers that retain the larger alkyl gro... 

De-O-protection. Both tetrahydropyranyl ethers and p-methoxybenzyl ethers are said to be cleaved with this reagent. However, the method does not seem to have any advantage over existing procedures. 

Photochemical addition of a formamide molecule onto the double bond of the 3-(/3-D-ribofuranosyl)acrylic ester 76 gave the succinamic ester 77 that cyclized and de-O-protected to a mixture of (/ )- and (5)-dihydroshow-domycin (78) (80MI11) (Scheme 13). 

The /3-anisyltelluro-D-ribofuranoside (101) photolytically generated the corresponding ribofuranosyl free radical, which stereoselectively coupled with maleimide to give, after dehydrogenation and de-O-protection, show-domycin (4) (90JA891) (Scheme 19). 

The 2,5-anhydro-D-allononitrile V-sulfide 556, obtained by the thermolysis of the 5-(/l-D-ribofuranosyl)-l,3,4-oxathiazol-2-one 555 (Section XXI Scheme 213), underwent 1,3-dipolar cycloaddition with ethyl acetylenecar-boxylate to give, after de-O-protection and amidation, a mixture of the... 

O-Protected l-deoxy-l-thiocyanato-2-ketoses such as 631 gave, upon de-O-protection, 5-(alditol-l-yl)thiazolin-2-ones (636) (94MI6) (Scheme 168). 

The aldol reaction of 282 with 288 was accomplished by use of dibutylboron triflate to give three diastereomeric adducts. One of them was de-O-protected to give an aglycone of elaiophylin, ll,ir-di-0-methylelaiophylidene (289), which was identical with a sample prepared by methanolysis of elaiophylin (284). 

A cross-coupling reaction of an o-iodoaniline and an alkyne is a key step in Merck s synthesis of MK-0462, a 5-HTid receptor agonist and potential antimigraine drug (Scheme 11). As a catalyst, simple palladium(ll) acetate, although in relative high concentrations (2 mol %), is used without any ligand. Under these conditions, 80% of the (partially de-O-protected) substituted indole is formed in DMF at 100... 

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