Acetonide derivatives

Acetonide Derivative 11. Benzylidene Acetal 13. p-Methoxybenzylidene Acetal 18. Methoxymethylene Acetal 20. Dimethoxymethylenedioxy Derivative... 

Previous syntheses of terminal alkynes from aldehydes employed Wittig methodology with phosphonium ylides and phosphonates. 6 7 The DuPont procedure circumvents the use of phosphorus compounds by using lithiated dichloromethane as the source of the terminal carbon. The intermediate lithioalkyne 4 can be quenched with water to provide the terminal alkyne or with various electrophiles, as in the present case, to yield propargylic alcohols, alkynylsilanes, or internal alkynes. Enantioenriched terminal alkynylcarbinols can also be prepared from allylic alcohols by Sharpless epoxidation and subsequent basic elimination of the derived chloro- or bromomethyl epoxide (eq 5). A related method entails Sharpless asymmetric dihydroxylation of an allylic chloride and base treatment of the acetonide derivative.8 In these approaches the product and starting material contain the same number of carbons. 

The absolute stereochemistries for both compounds were determined by CD analysis of the p-bromobenzoate derivatives. Two other eicosanoids were also isolated from whole animals [198], The trihydroxylated oxylipin 8jR,11S,12R-trihydroxyeicosa-5Z,9 ,14Z,17Z-tetraenoic acid (trioxilin A4,150) was detected in, or isolated from, five starfish species P. miniata, Dermasterias imbricata, Pycnopodia helianthoides, Culcita novaeguinea, and Nardoa tubercolata. Its structure was determined by H and 13C NMR and FAB-MS analyses of the natural product and of an acetonide derivative. The co6 analog of this compound (151) was isolated only from P. helianthoides. The relative stereochemistry in these metabolites (150, 151) was established from a comparison of NMR shifts with malyngic acid, a trihydroxylated C18 compound isolated from Lyngbya majuscula, while the absolute stereochemistry was proposed on the basis of the earlier isolation of 8R-HETE from P. miniata. 

The above interesting approach to the asymmetric allyltitanation reaction does, however, have a limitation. Thus, L-glucose is much more expensive that the D-form and, consequently, homoallylic alcohols of the opposite configuration cannot easily be obtained by this method. In an attempt to induce the opposite si face selectivity, other acetonide derivatives of monosaccharides from the xylose, idose, and allose series were tested [42b,42c], The enantiofacial discrimination was, however, much lower than that with DAGOH and both re and si face selective additions to aldehydes were observed. 

For 8-NH-Boc-Y-mesyloxy-a, 3-enoates, only anti-SN2 substitution has been observed. However, when the NH is absent, such as in Pro-derived enoates, or Ser and Thr acetonide-derived enoates 88 or 89 (Scheme 17), the syn-SN2 stereochemistry has also been observed. 93 ... 

T., Malone, J.F., McGregor, S. and Allen, C.C.R. (2005) Dioxygenase-catalyzed dihydroxylation of arene cis-dihydrodiols and acetonide derivatives a new approach to the synthesis of enantio-pure tetraoxygenated bioproducts from arenes. Chemical Communications, 4934-4936. 

The reagent has proved useful as water scavenger in the preparation of acetonide derivatives of nucleosides see Di-p-nitrophenyl hydrogen phosphate). 

Attention has been drawn to the remarkable stability of 21-bromo-17a,20-acetonides, but debromination occurred with lithium aluminium hydride and with sodium in propan-2-ol, although the main products with the latter reagent were the two olefins (534) and (535). Similar 17a,20-acetonides derived from cortisone have been prepared and oxidised with chromium trioxide in pyridine to the corresponding 21-aldehydes and 21-carboxylic acids. 

A further example of a highly diastereoselective hydrogenation involves the acetonide derivative 9 which provides a rigid framework with a strong facial bias for / -attack on the exocyclic double bond. Hydrogenation over platinum oxide leads to the a-methyl isomer 10 as the sole product19. 

Scheme 4 Compound (45) prepared from a-santonin (41), on reduction afforded alcohol (46), which was converted to derivative (48) by standard organic reaction. Irradiation of (48) in benzene at room temperature followed by reflux in tetrahydrofuran and 2-propanol, produced 15-oxime derivative (49). Its conversion to nitrile (50) was achieved by treatmente with acetic anhydryde in pyridine. Dehydration of (50) followed by decyanation, yielded compound (52). Its acetonide derivative (53), was converted to compound (54), whose convertsion to rishitin (40), was achieved by heating with 0.5% PPA in ethanol...

In order to prevent the degradation of the somewhat labile cw-diols upon storage, they were converted to their acetonide derivatives by treatment with 2,2-dimethoxypropane (2,2-DMP) and Amberlite 120-11 resin, followed by filtration and removal of solvents. Care needed to be taken in order to minimize losses of the somewhat volatile silanes during the concentration step (Table 3). 

It has been proposed that hydrogen bonding between the 12a-F and 17a-0H groups renders the analogue inactive (Fig. 33.16). Conversion to the 16a,17a-isopropylidinedioxy (acetonide) derivative, which cannot hydrogen bond, restores biological activity (119). 

Taurospongin A (78) contains three chiral centers at C-3, C-7, and C-9. The relative stereochemistry of 1,3-diol at the C-7/C-9 position was revealed to be syn by applying Rychnovsky s C-acetonide analysis [51] for the acetonide (82 oc 19.9, 30.3, and 98.4) prepared fi-om 81. To elucidate the relative stereochemistry between C-3 and the C-7/C-9 position as well as the absolute stereochemistry of these three chiral centers, two possible diastereomers (83 and 84), which correspond to the acetonide derivative (82), were prepared as optically active forms as shown in Scheme 7. The methoxymelhyl (MOM) ether (86) derived from (/ )-3-hydroxy-3-methyl-5-pentanolide (85) was reduced with DIBAL to give a lactol, which was subjected to Wittig reaction with chloromethylenetriphenylphosphorane followed by protection with benzyloxymethyl group to yield a 2 1 mixture of the E/Z chlorovinyl... 

Acetone reacts with carbohydrates in the presence of an acid catalyst to form products that are commonly referred to as isopropylidene or acetonide derivatives. The carbohydrate D-ribono-(l,4)-lactone reacts with acetone in the presence of hydrochloric... 

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