1.5- Diols pyrans, tetrahydro

Diels-Alder reactions have featured heavily during the period of review. Ar-Vinyl-2-oxazolidinone has been reported as a dienophile for the first time, including the preparation of various tetrahydro-277,77/-pyrano[4,3-. ]pyrans <2002SL952>. The heterodiene cycloaddition reaction of 3-formylchromone with a series of ketene acetals formed from C2-symmetric l,2-diarylethane-l,2-diols is completely diastereoselective (Scheme 40) <1995J(P1)2293>. 

Dispirodiketals can be introduced by reaction of a 1,2-tranr-diol with 3,3, 4,4 -tetrahydro-6,6 -bi-2H-pyran in the presence of a catalytic... 

On the other hand, higher 1,2-asymmetric induction is observed for the cyclization of 5-aIkenols which bear an oxygenated function at C-4. Thus, treatment of 5-hexen-1,4-dioI (3) with phenyl-selenyl triflate in dichloromcthanc at —78 °C gives the corresponding tetrahydro-2/f-pyran 4 in 52% yield as a 91 9 (cis/trans) mixture. The preferential 2,3-cis relationship can be explained by analogy with the halocyclization of 4-pentene-l,3-diols (see Section 4.6.2.1.1.3.)19. 

Moreover, starting from 2,9-dimethyl-10-undecene-4,6-diol (3), the alkoxycarbonylation reaction results in the corresponding tetrahydro-2//-pyran 4 in moderate yield and a cis/trans ratio of 89 1183. 

IVvo new methods have been discovered for Ihe selective protection of rrans-dieqnatoiial vicinal diols [44,45]. One of them uses 33, 4,4 -tetrahydro, 6 -hi-2H-pyran (bis-DHP) 40 to transform diols into dispiroacetals and the second uses l,l,2,2-tetrahydr(Mnedioxycyclo-Imxane 41 to obtain cyclohexane-l,2-diacetal-protected sugars. These two methods have been compared [44,45] and we pve hoe, as rquesmtative examples, one procedure for each strategy. 

The dispiroketal protection of monosaccharides is controlled by the stabilizing influence of multiple anomeric effects leading to a single diastereomeric derivative. In certain examples, where there is more than one diequatorial diol pair present in the molecule, as for example in D-glucose derivatives, reaction affords a mixture of diacetals (O Scheme 22). The reaction, often giving crystalline compounds, is carried out by treatment of the polyol with 3,4,3, 4 -tetrahydro-6,6 -bis-2H-pyran in chloroform at reflux in the presence of a catalytic amount of CSA[151]. 

Because of their kinship to syntheses of nitro sugars, some syntheses of simpler, cyclic, aliphatic nitro compounds will be mentioned. Tetrahydropyran-2-ol, which is in tautomeric equilibrium with 5-hydroxypentanal (126), was found to undergo nitromethane addition catalyzed by sodium hydroxide to give120 tetrahydro-2-(nitromethyl)-pyran (128). Undoubtedly, the first reaction-product is the diol ni-tronate (127), which, under the conditions of processing (acidification and steam distillation), forms the cyclic ether 128. 

Optically active tetrahydropyran derivatives can be obtained by domino cross aldol/acetal cyclization reaction of aromatic aldehydes with glutaraldehyde generated from the inexpensive tetrahydro-2/f-pyran-2,6-diol under equilibrium conditions, in yields ranging from 42% to 78% and good diastereo- (60-75% de) and enantioselectivities (93-99% ee) [75],... 

With dialdehydes as enol components, the aldol reaction may be followed by an acetalization event Thus, Hayashi and coworkers [10] established a highly enan-tioselective proHne-catalyzed domino aldol-acetalization of aqueous tetrahydro-2H-pyran-2,6-diol (21) and aldehydes (Scheme 8.7). In this sequence, the intermediate aldolate product 22 is trapped by the additional aldehyde moiety to generate a lactol, which through addition of methanol and acid is converted into the double acetal 23. 

Benzoyloxy)methyl]-7-oxabicyclo[4.1.0] hept-3-ene-2,5-diol 5-benzoate, see P-30114 7-[(Benzoyloxy)methyl]-l, 4a,5,7fl-tetrahydro- 5-hydroxycyclopenta[c]pyran-1 -yl-/i-D-glucopyranoside, see M-10026 Benzoyloxypaeoniflorin, in 0-20066 A-Benzoyl-A-phcnylaminomethylcarbinoK H-10135... 

The vinylsulphones (27) undergo an intramolecular Michael reaction in the presence of a catalytic amount of potassium hydride giving tetrahydrofurans in high yield (>8l%), and the diols (28) can be cycllzed to the corresponding tetrahydro-furans or -pyrans... 

Tetra-O -beiizoyl-3-deoxy-p-D-e 7i/iro -hex-2-nlopyranose, D-219 Tetrahydro-3-hydroxy-5-hydroxymethyl-3-furancarboxylic acid, T-26 Tetrahydro-2/7-pyran-2,3-diol Q.BS, iRSyfomr. 2-Me ether, 3-Ac, T-28 Tetrahydro-2/7-pyran-2,3-diol Q.RS, iRSyform 2-Me ether, T-28... 

In 1992, Ley and co-workers demonstrated the inherent selectivity of 3.3, 4.4 -tetrahydro-6,6 -bis-2//-pyran (bis-DHP) 1 for trans diequatorial vicinal diols in polyol systems in carbohydrate derivatives (Scheme 1) [26]. In a typical experiment, the carbohydrate polyol was reacted with an excess of bis-DHP 1 in refluxing chloroform and in the presence of a catalytic amount of camphorsulfonic acid to afford the corresponding dispiroketal. For representative examples see Table 1. The protection process proceeds in moderate to good yields and gives diequatorial diol protection as the major outcome in all cases. In a few cases some c/s-diol protection was noticed as a minor process when steric interactions were of lesser magnitude. 

A -(para-methoxyphenyl) aldimines and inexpensive aqueous tetrahydro-2-/f-pyran-2,6-diol. ° These excellent results are collected in Scheme 3.6. 

Small amount of water (10-50 equiv.) are able to affect the stereoselectivity of the products between 9-tosyl-3,4-dihydro-P-carboline and ketones in the presence of L-proline as a catalyst in DMSO (Scheme 24.5a) [131]. This effect was also observed in CH2CI2 and acetone, although it cannot be fully rationalized. Later, proline gave rise to high stereoselectivities in the catalytic reaction of aqueous tetrahydro-2H-pyran-2,6-diol with N-(p-methoxyphenyl) (PMP) aldimines in DMSO to afford the corresponding tetrahydropyridines via a Mannich-type/ intramolecular cycUzation cascade reaction (Scheme 24.5b) [132]. 

The tetrahydropyranyl derivative of propaigyl alcohol, tetrahydro-2-(2-propynyloxy)-2/f-pyran, can be converted to methyl 4-hydroxy-2-butynoate in four steps. Diethyl [(2-tetrahydropyranyloxy)methyl]phosphonate is a convenient Wadsworth-Emmons reagent. Tetrahydropyranyl esters of a-bromo acids can be used in the Reformatsky reaction for the preparation of (3-hydroxy acids. Elimination of a tetrahydropy-ranyloxy moiety from butyne-l,4-diols with lithium hydride constitutes an efficient method for the synthesis of allenic alcohols. ... 

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