Practical asymmetric synthesis of aklavinone

1-silyl-2,6-diketones: versatile intermediates for the divergent synthesis of five- and six-membered carbocycles under radical and anionic conditions

1-Silyl-2,6-diketones, readily prepared by addition of (silylmethyl)metal reagents to 1,5-lactols followed by oxidation of the resultant diols, can be efficiently transformed into 3-hydroxycyclohexanones, cyclohex-2-enones, or 1-(silylmethyl)cyclopentane-1,2-diols under nucleophilic, basic, or single electron-transfer reduction conditions, respectively. The latter cyclitols can be further transformed into 2-methylenecyclopentanols or 1-(hydroxymethyl)cyclopentane-1,2-diols by Peterson elimination or Tamao-Fleming oxidation, respectively.

Mild and Efficient Removal of Hydroxyethyl Unit from 2-Hydroxyethyl Ether Derivatives Leading to Alcohols

[reaction: see text]. CAN is a good reagent for the transformation of 2-hydroxyethyl ether units to alcohols. Significantly, many functional groups can tolerate the reaction conditions, although they do not survive under many previously reported removal conditions. The reaction mechanism is clarified.

Universal NMR Databases for Contiguous Polyols

On the basis of 1,2,3-triols 1a approximately d, 1,2,3,4-tetraols 2a approximately h, and 1,2,3,4,5-pentaols 3a approximately p, NMR databases with four types of profile-descriptors ((13)C-, (1)H-, and (1)H(OH)-chemical shifts and vicinal spin-coupling constants) for contiguous polyols are reported. To systematically assess the relative values of these databases, a case study has been conducted on heptaols 4a approximately d, through which the gamma- and delta-effects have been recognized to refine the (13)C and (1)H chemical shift profile predicted via an application of the concept of self-contained nature. The magnitudes of gamma- and delta-effects depend on a specific stereochemical arrangement of the functional groups present in both the inside and outside of a self-contained box and are significant only for the stereoisomers belonging to a specific sub-group. With the exception of the stereochemical arrangement of functional groups belonging to a specific sub-group, the gamma- and delta-effects can, at the first order of approximation, be ignored for the stereochemical analysis of unknown compounds. For the stereoisomers belonging to a specific sub-group, it is necessary to refine, with incorporation of the gamma- and delta-effects, the profile predicted at the first order of approximation. With use of heptaols 4a approximately d, the values of (3)J(H,H) profiles have been assessed. Two methods, one using profiles consisting of three contiguous (3)J(H,H) constants and the other using profiles consisting of two contiguous (3)J(H,H) constants, have been developed. A stereochemical analysis based on three, or two, contiguous (3)J(H,H) profiles is operationally simpler than one based on (13)C and (1)H chemical shift profiles. Therefore, it is recommended to use a (3)J(H,H) profile as the primary device to predict the stereochemistry of unknown polyols and (13)C and (1)H chemical shift profiles as the secondary devices to confirm the predicted stereochemistry.

A highly stereoselective approach to the synthesis of functionalized pyran derivatives by Lewis acid assisted ketal reduction and allylation

Reduction of bicyclic ketal 1 gave functionalized pyran derivatives 7a or 7b in a highly stereoselective manner, depending upon the reduction conditions utilized. For example, treatment of ketal 1 with TiCl4/Et3SiH produced exclusively diol 7b with the 2,5-syn relationship in good yield. Alternatively, reduction of ketal 1 by DIBALH gave 2,5-anti-diol 7a stereoselectively. Alane reductions of ketal 1 were highly stereoselective also; however, the syn/anti selectivity observed was strongly dependent on the ratio of reagents employed for in situ generation of the alane. Lewis acid catalyzed allylation of ketal 1 gave pyran 10 in a stereospecific alkylation reaction.

Synthesis of enantiopure homoallylic ethers by reagent controlled facial selective allylation of chiral ketones

The stereoselective allylation of chiral methyl ketones to give tertiary homoallylic ethers, which can easily be transformed into homoallylic alcohols, is described. Reaction of the enantiopure ketones 8a-d and the racemic ketones 26a-d with the norpseudoephedrine derivative 2 or ent-2 and allylsilane in the presence of a catalytic amount of trifluoromethanesulfonic acid, led to a series of homoallylic ethers with good to excellent diastereoselectivity (85:15 to > 97:3). The allylation is reagent controlled and nearly independent from the stereogenic centers in the substrates. A partial kinetic resolution was observed using the racemic ketones 26a-d. In the reaction of the chiral ketones 8a-d with the achiral reagents ethoxytrimethylsilane and allylsilane only a low diastereoselectivity was observed.

Synthesis (and alternative proof of configuration) of the scyphostatin C(1')-C(20') trienoyl fragment

[reaction--see text] Each of four diastereomers of structure 2, corresponding to the lipophilic side chain of scyphostatin (1), were prepared. Careful analysis of their NMR spectral data and comparison with those of the natural product corroborates the recently reported (Org. Lett. 2000, 2, 505) stereochemical assignment. A strategy for the stereoselective synthesis of 2 has been achieved.

Toward creation of a universal NMR database for the stereochemical assignment of acyclic compounds: the case of two contiguous propionate units

[formula: see text] Using triol 1 as a representative example of natural products containing two contiguous propionate units, 13C and 1H NMR databases for the stereochemical assignment of acyclic compounds have been created. Chemical shift increments due to the presence of additional functional groups as well as solvent effects are discussed.

Stereochemical Determination of Acyclic Structures Based on Carbon-Proton Spin-Coupling Constants. A Method of Configuration Analysis for Natural Products

A method for elucidating the relative configuration of acyclic organic compounds was developed on the basis of carbon-proton spin-coupling constants ((2,3)J(C,H)) and interproton spin-coupling constants ((3)J(H,H)). This method is based on the theory that, in acyclic systems, the conformation of adjacent asymmetric centers is represented by staggered rotamers, and their relative stereochemistry can be determined using (2,3)J(C,H) and (3)J(H,H), because the combined use of these J values enables the identification of the predominant staggered rotamer(s) out of the six possible derived from threo and erythro configurations. Detailed conformational analysis for model compounds 1-4 revealed that this method is useful in most cases for assignment of the configuration of acyclic structures occurring in natural products, in which stereogenic methine carbons are often substituted with a methyl or a hydroxy (alkoxy) group. This J-based configuration analysis was applied to the stereochemical elucidation of carboxylic acid 5 derived from zooxanthellatoxin and proven to be a practical method even for natural products with complicated structures.

A Remarkable Effect of C-O-C Bond Angle Strain on the Regioselective Double Nucleophilic Substitution of the Acetal Group of Tetraacetal Tetraoxa-Cages and a Novel Hydride Rearrangement of Tetraoxa-Cages

A remarkable effect of C-O-C bond angle strain on the regioselective double nucleophilic substitution of the acetal group of tetraacetal tetraoxa-cages and a novel regioselective and stereoselective hydride rearrangement of tetraoxa-cages are reported. Reaction of the tetraacetal tetraoxa-cages 1 with 3 equiv of triethylsilane (at -78 degrees C), cyanotrimethylsilane (at 25 degrees C), and allyltrimethylsilane (at -78 degrees C) in dichloromethane in the presence of TiCl(4) gave the double nucleophilic substitution products 2, 6, and 7 in 85-90% yields, respectively. No detectable amount of other regioisomers was obtained. Reaction of 1a with (methylthio)trimethylsilane and (phenylthio)trimethylsilane in dichloromethane in the presence of TiCl(4) at -78 degrees C gave the symmetric products 10a,b and the unsymmetric products 11a,b in ratios of 8-10:1. The stereochemistry of the symmetric substitution products was proven by X-ray analysis of the crystalline compound 10a. The mechanism of the double nucleophilic substitution of the tetraoxa-cages 1 are discussed. Treatment of the tetraoxa-cages 1a,c and 22a-c with 2 equiv of TiCl(4) or MeSO(3)H in dichloromethane at 25 degrees C for 3 h regioselectively and stereoselectively gave the novel hydride rearrangement products 16a,b and 23a-c respectively. No detectable amount of other regioisomers was observed. The stereochemistry of the hydride rearrangement was proven by DIBAL-H reduction of 16 and 23 and X-ray analysis of the reduction product 24a. We attribute the high regioselectivity of the double nucleophilic substitution and the hydride rearrangement of the tetraoxa-cages 1 to the bond angle strain of the unusually large bond angle of C(3)-O(4)-C(5) of the tetraoxa-cages.