Acetonide protection with

In all cases examined the ( )-isomers of the allylic alcohols reacted satisfactorily in the asymmetric epoxidation step, whereas the epoxidations of the (Z)-isomers were intolerably slow or nonstereoselective. The eryfhro-isomers obtained from the ( )-allylic alcohols may, however, be epimerized in 95% yield to the more stable tlireo-isomers by treatment of the acetonides with potassium carbonate (6a). The competitive -elimination is suppressed by the acetonide protecting group because it maintains orthogonality between the enolate 7i-system and the 8-alkoxy group (cf the Baldwin rules, p. 316). 

In the following example the acetonide protective group is selectively converted to one of two t-butyl groups. The reaction appears to be general, but the alcohol bearing the t-butyl group varies with structure.Benzyli-dene ketals are also cleaved. 

Selective protection of 1,2- and m-l,3-diols can be achieved by formation of acetonides, acetals or orthoesters. Further selectivity is possible in special cases (e.g., acetonide formation). With 17a,20,21-triols, the 20,21-acetonide is obtained exclusively. 16a,17a,21-Trihydroxy-20-lcetopregnanes (20) react selectively with acetone to give 16,17-acetonides (21). 

Intermediate 8, the projected electrophile in a coupling reaction with intermediate 7, could conceivably be derived from iodolactone 16. In the synthetic direction, cleavage of the acetonide protecting group in 16 with concomitant intramolecular etherification could result in the formation of the functionalized tetrahydrofuran ring of... 

Alkylation of cyanohydrin acetonide 79 with the iodide 78 proceeded smoothly to give pentaacetonide 80 in 70% yield (Scheme 10). This represents the entire polyol framework of roflamycoin. An eight-step sequence involving installation of the polyene, macrocyclization via Horner-Emmons reaction, and protecting group machinations, completed the first total synthesis of roflamycoin. 

As part of a strategy of employing monosaccharides as a convenient source of chirality, organometallic additions to protected L-erythrulose derivatives have been carried out. Employing silyl, benzyl, trityl, and acetonide protecting groups, the diastereoselectivities obtained are discussed in terms of chelation to the a-and/or the /3-oxygen, and are compared with results for similar aldehydes. 

Alternatively, epoxide (3R,5S)-14 was opened regioselectively with lithium iodide on silica [29], The crude product was immediately subjected to acetonide protection, which afforded the desired iodide sy -(3R,5S)-13 with a 58% yield (44% from syn-(3R,5S)-5a Scheme 2.2.7.7). Epoxide (3R,5S)-14 was easily obtained from dihydroxy ester syn-(3R,5S)-5a by treatment with DBU (66% yield) [11]. Treatment of sy -(3R,5S)-5a with LiCN in CH2CI2 gave nitrile (3R,5R)-15 in almost quantitative yield [21]. 

A variation of this route was applied to the preparation of a-methylenecyclo-pentane 179, an intermediate that was employed for the synthesis of prostaglandin PGF2o, (180) (Scheme 6.82). The acetonide-protected oxime-diol 175 [derived from propanal (174)] was treated with sodium hypochlorite without the addition of base. This led to the tricyclic adduct 176 with high stereoselectivity. One of the side chains was subsequently elaborated and the fully protected cyclopentano-isoxazo-line (177), when exposed to Raney Ni/boron trichloride, gave the 2-hydroxymethyl-cyclopentanone (178). This compound was dehydrated using mesyl chloride/ pyridine to furnish enone (179) (324). In another related synthesis of PGF2q, the p-side-chain (3-hydroxyoctenyl) was introduced prior to the cycloaddition (325). 

Cyclization of the bis-epoxides 123 (mixtures of diastereomers) with benzylamine afforded the azepanes 124 in a preferential 1-endo-tet methodology (Equation 17) <1995JOC5958> however, with the corresponding 3,4-benzyl ethers of the bis-epoxides rather than the // / -acetonide protecting group, no cyclization to six-membered ting species was observed. The azepanes 124 were then converted in two steps to the amino sugar hydrochloride salts 126 via the N-debenzylated intermediates 125 (Scheme 16). 

A similar observation was made by Marco and coworkers when synthesizing microcarpalide [29]. In the case of the cyclization of acetonide 36, the Z-olefin is the thermodynamic product, produced with the second-generation catalyst G2, while use of Gl gives a 2 1 mixture of E/Z alkenes 37 (Scheme 2.14). These compounds could be separated by chromatography and the E-isomer was subsequently converted to microcarpalide by removal of the MOM and the acetonide protecting groups. 

After extensive optimization of the reaction conditions regarding yield as well as diastereo- and enantioselectivity, we were able to obtain the aldol product 26 with 60% yield and excellent diastereo- and enantiomeric excesses (>99% de, 95% ee). Thus, the simple (S)-proline-catalyzed aldol reaction of 4 with pentadecanal directly delivered gram-amounts of the selectively acetonide protected ketotriol precursor 26 of the core unit of phytosphingosines in excellent stereoisomeric purity (Scheme 7). 

ADMET polymers containing D-chiro-inositol have been prepared by polymerizing the acetonide-protected inositol diene followed by deprotection to the D-c/z/ro-inositol containing polymer [171]. Synthesis of the monomer precursor was performed using whole-cell fermentation of bromobenzene with... 

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