Arabinose, protected

As interesting synthetic approaches toward lycoricidine, starting with optically active tribenzyloxy-5-hexenal or L-arabinose, protected versions 2 (151) or 228 (152) of (+)-lvcoricidine have been synthesized through 225 or 227 (Fig. 15). 

Replacement of heterocyclic rings in nucleosides by ring systems which do not occur in nature represents another approach to compounds which may have activity against viral and neoplastic diseases. One of the early successes in this category involves replacement of a pyrimidine ring by a triazine. The synthesis starts with a now classical glycosidation of a heterocycle as its silylated derivative (146) with a protected halosugar (145), in this case a derivative of arabinose... 

D-Ribono-1,4-lactone (1) readily condenses with acetone, under acidic catalysis with mineral acids or anhydrous copper sulfate, to give 2,3-0-isopropylidene-D-ribono-1,4-lactone (16a), which was employed for the synthesis of 5-deoxy and 5-0-substituted derivatives of D-ribono- 1,4-lactone and D-ribitol (24). Acid removal of the 1,3-dioxolane protecting group gave products having probable inhibitory activity of arabinose 5-phosphate isomerase (25). Other applications of 16a for the synthesis of natural products will be discussed later. 

C]gulono- 1,4-lactone. The hydroxyl groups at C-2 and -3 were protected by isopropylidenation, and the 5,6-glycol was oxidized by sodium periodate. Treatment of the resulting syrupy product with methanolic hydrogen chloride, followed by borohydride reduction and hydrolysis, afforded L-[5-,4C]arabinose. 

The transformation of L-arabinose (58) to lactone 57 was based on a route developed by Marquez and Sharma [51] Selective protection of the primary hydroxy group with TBDPSCl and oxidation of the lactol moiety with bromine afforded lactone 59. Subsequent selective deoxygenation a to the carbonyl group proceeded under Barton-McCombie conditions providing lactone 57 in 21% yield (Scheme 14). 

Since L-arabinose (15) and o-xylose (16) (Fig. 8) are easily extracted from wheat straw and bran [26, 27], the telomerization of 1 with these pentoses constitutes an attractive route to prepare surfactants without the use of expensive protection and deprotection steps. 

Several 2,5-anhydropentitols have been synthesized from suitably protected pentoses by standard procedures (see also, Ref. lb). Thus, ethyl 5-O-p-tolylsulfonyl-a-L-arabinofuranoside (11) was converted17 into the corresponding 2,5-anhydride (12) by treatment with metha-nolic sodium methoxide. The product was hydrolyzed, and the resulting 2,5-anhydro-L-arabinose was hydrogenated in the presence of Raney nickel to give 2,5-anhydro-L-arabinitol (13) as a syrup. 

The aminopolyol adducts obtained with carbohydrates can be manipulate further to produce aminosugars (Scheme 7.27) [26, 75]. For example, starting with D-arabinose 207 and a cleavable amine, such as 208 or 213, followed by conversion to a protected aminopolyol 211 and ozonolysis of the alkene forms the protected mannosamine 212 in good overall yield and purity. 

The chiral bicyclic enones, levoglucosenone, isolevoglucosenone, and new functionalized L-arabinose enone possess excellent reactivity and functionality. Their properties mid application as convenient precursors in the synthesis of many attractive templates or intermediates of complex natural products are reviewed. These compounds are attracting increasing interest due to their structural rigidity and ability for stereoselective functionalization without protection, deprotection sequences necessary in many synthetic organic methodologies. 

Natural L-arabinose as one of the highly functional pentoses with four chiral centers with different reactivities of secondary hydroxyls is an excellent precursor for the selective functionalization. Klemer and coworkers (31) synthesized one of the first valuable arabinose building block with protected C-l and C-2 hydroxyl group and a conjugated enone between C-3, C-5 (scheme 12). This highly reactive enone should have synthetic potential through the introduction of additional functional groups at either C-3 or C-5. 

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