Pyranose 2,3-unsaturated

The q cloaddition reaction catalyzed by complex 9d proceeds well for several aromatic and a,/ -unsaturated aldehydes and has been used in an enantioselective route to the carbon-branched pyranose derivative cis-3c (Scheme 4.10) [17]. 

If the carbonyl and the hydroxyl group are in the same molecule, an intramolecular nucleophilic addition can take place, leading to the formation of a cyclic hemiacetal. Five- and six-membered cyclic hemiacetals are relatively strain-free and particularly stable, and many carbohydrates therefore exist in an equilibrium between open-chain and cyclic forms. Glucose, for instance, exists in aqueous solution primarily in the six-membered, pyranose form resulting from intramolecular nucleophilic addition of the -OH group at C5 to the Cl carbonyl group (Figure 25.4). The name pyranose is derived from pyran, the name of the unsaturated six-membered cyclic ether. 

A similar approach had been described for the synthesis of the mycotoxin patulin (2,3-pyranose-fused unsaturated lactone) from a 2-arabinopyranos-2-uloside derivative,16 and more recently, for the synthesis of a potent germination stimulant and some analogous compounds from D-xylose or commercially available D-glucuronic acid y-lactone.17... 

Aiming at the pyranose form of sugars, normal type hetero-Diels-Alder reactions were extensively used for the synthesis of functionally substituted dihydropyran and tetrahydropyran systems (5-10) (see routes A - D in the general Scheme 1) which are also important targets in the "Chiron approach" to natural product syntheses (2.) Hetero-Diels-Alder reactions with inverse electron demand such as a, p-unsaturated carbonyl compounds (l-oxa-1,3-dienes) as heterodienes and enol ethers as hetero-dienophiles, are an attractive route for the synthesis of 3,4-dihydro-2H-pyrans (11). 

P2i2i2i Z = 4 Dx = 1.528 R = 0.095 for 1,067 intensities. The conformation of the unsaturated pyranose is °HS. The attachment of the iodine atom and of the ethoxyl group is quasi-axial, and that of the benzoate group is quasi-equatorial. The benzoate group is planar, and is approximately perpendicular to the pyranose ring, as defined by the torsion angles 0-5-C-5-C-6-0-6, C-5-C-6-0-6-C=0 of +74°,... 

Fraser-Reid s stereocontrolled synthesis of the Woodward reserpine precursor 195 relied upon a tandem 5-exol6-exo radical cyclization of pyranose-derived dienes [76-77]. As outlined in Scheme 36, a,P-unsaturated ester 188 was prepared by free radical coupling of iodide 187 with a tin acrylate. After hydrolysis of 188 (MeONa, MeOH, 100%) to give primary alcohol 189, the silicon tethered diene 190 was installed by silylation. Treatment of 190 with n-BujSnH led to the desired cage molecule 192 in high yield via a 5-exo-trig cyclization to intermediate 191 followed by a 6-exo cyclization. Tamao oxidation of tricycle 192 led to diol... 

In addition to the pyranose (54) and the bicyclic (55) forms, the unsaturated compound 53 takes part in the equilibrium of 5,6-diamino-... 

Unsaturated derivatives of cellulose containing double bonds directly in the pyranose ring have b n synthesized by the thermal decomposition of cellulose methylxanthate or bis-xanthate (39,40), which proceeds as follows ... 

Coupled with his achievements in carrying out highly regio and stereo selective additions to glycals (1,2-unsaturated pyranose derivatives) to give glycosides of 2-amino-2-deoxy sugars, NMR spectroscopy enabled Lemieux to lead the subject into the next phase of its development. 

In this section the selected and most representative examples (including also several older ones) of the preparation of unsaturated monosaccharides with the double hond present in the sugar ring will be described. The material will be divided into three main classes 2,3-, 3,4-, and 4,5-unsaturated sugars in both pyranose and furanose forra... 

The 4,5-unsaturated pyranoses or 3,4-unsaturated furanoses are usually prepared by a base-catalyzed elimination of a leaving group such as halogen, sulfonyloxy etc. from appropriate sugar derivatives. The classical example is represented by the synthesis of compound 19 from l,2 5,6-di-0-isopropylidene-a-D-glucofuranose 18 [1]. Analogous elimination performed for 20 led to derivative 21 from which capuramycin—a complex nucleoside antibiotic could be prepared readily (O Scheme 14) [30]. 

During a study devoted to fragmentation of sugar acetals in the presence of butyllithium [142], the formation of unsaturated pyranoses (36-38) from L-rhamnose derivative 35 was observed and explained in terms of anionic species rearrangement (O Scheme 12). 

Esterified glycals 81 easily undergo reaction with nucleophiles, typically under Lewis acid catalysis to 2,3-unsaturated pyranoses 82. This reaction is known as Perrier rearrangement [7]. 

In the retrosynthetic plan (Scheme 14), the key step was based on the crucial supposition that the truns-fused bicyclic hemiacetal Dill would undergo a reversible ring tautomerization with the monocyclic system DII, the skeletal and functionalization of which are well correlated with both actinobolin 1 and bactobolin 2 (vide infra). Further retrosynthesis of Dill led to unsaturated hemiacetal DIV which is reminiscent of a 2,3-unsaturated pyranose (i.e., a pseudo glycal) [46], and thence to lactone DV. The construction of the unsaturated bicyclic lactone therefore constituted the first plateau of this synthesis. 

---