Erythrulose as a multifunctional chiron: Highly stereoselective boron aldol additions

Total Synthesis of Aflastatin A

The total syntheses of aflastatin A and its C3-C48 degradation fragment (6a, R = H) have been accomplished. The syntheses feature several complex diastereoselective fragment couplings, including a Felkin-selective trityl-catalyzed Mukaiyama aldol reaction, a chelate-controlled aldol reaction involving soft enolization with magnesium, and an anti-Felkin-selective boron-mediated oxygenated aldol reaction. Careful comparison of the spectroscopic data for the synthetic C3-C48 degradation fragment to that reported by the isolation group revealed a structural misassignment in the lactol region of the naturally derived degradation product. Ultimately, the data reported for the naturally derived aflastatin A C3-C48 degradation lactol (6a, R = H) were attributed to its derivative lactol trideuteriomethyl ether (6c, R = CD₃). Additionally, the revised absolute configurations of six stereogenic centers (C8, C9, and C28-C31) were confirmed.

Conformationally Biased Ketones React Diastereoselectively with Allylmagnesium Halides

The addition of the highly reactive reagent allylmagnesium halide to α-substituted acyclic chiral ketones proceeded with high stereoselectivity. The stereoselectivity cannot be analyzed by conventional stereochemical models because these reactions do not conform to the requirements of those models. Instead, the stereoselectivity arises from the approach of the nucleophile to the most accessible diastereofaces of the lowest-energy conformations of the ketones. High stereoselectivity is expected, and the stereochemical outcome can be predicted, with conformationally biased ketones that have sterically distinguishable diastereofaces wherein only one face is accessible for nucleophilic addition. The conformations of the ketones can be determined by a combination of computational modeling and, in some cases, structure determination by X-ray crystallography.

One-Pot Domino Aldol Reaction of Indium Enolates Affording 6-Deoxy-α-D,L-altropyranose Derivatives: Synthesis, Mechanism, and Computational Results

The domino-aldol-aldol-hemiacetal-reaction cascade of indium and other group 13 metal enolates furnished 6-deoxy-α-D,L-altropyranose derivatives in up to 99% yield under thermodynamic control. At lower temperature and thus under kinetic control, the reaction proceeded in a much less diastereoselective manner. The changeover from kinetic to thermodynamic control operating in this multistep domino-aldol-aldol-hemiacetal protocol was used for probing the efficiency of DFT computations. Calculations at the B3LYP/6-31G(d)/LANL2DZ level provided a mechanistic picture in full agreement with the experimental outcome.

Synthetic studies toward (-)-FR901483 using a conjugate allylation to install the C-1 quaternary carbon

Two approaches to the aza-tricyclo dodecane skeleton of (-)-FR901483 are reported. Both routes utilized a Grignard addition to an N-acylpyridinium salt to establish the absolute stereochemistry at C-6 and a highly diastereoselective conjugate allylation reaction to form the quaternary center at C-1 of the natural product in an excellent yield. Although the desired polysubstituted piperidine intermediates were prepared regio- and stereoselectively, the construction of the C-8/C-9 bond connectivity could not be achieved. All attempts at a pinacol cyclization or an intramolecular 6-exo-tet epoxide opening were unsuccessful because of an unfavorable A(1,3) strain inherent in the molecule.

Complex Aldol Reactions for the Construction of Dense Polyol Stereoarrays: Synthesis of the C33−C36 Region of Aflastatin A

[reaction: see text]. Facial selectivity in the addition of boron enolates of alpha-oxygenated ketones to anti-disposed alpha,beta-bisalkoxy aldehydes is controlled by the aldehyde vicinal diol protecting group. Protection of the diol as an acetonide results in the exclusive formation of the anti-syn-anti stereoarray found in the C(33)-C(36) region of aflastatin A.

Mimicking Dihydroxy Acetone Phosphate-Utilizing Aldolases through Organocatalysis: A Facile Route to Carbohydrates and Aminosugars

[reaction: see text] A practical and environmentally friendly organocatalytic strategy designed to mimic the DHAP aldolases has been developed and shown to be effective in the preparation of carbohydrates and aminosugars. (S)-Proline and (S)-2-pyrrolidine-tetrazole catalyzed the aldol reaction between dihydroxy acetone variants such as 1,3-dioxan-5-one and 2,2-dimethyl-1,3-dioxan-5-one with aldehydes to give the corresponding polyols in good yields with very high ees.

The Dihydroxyacetone Unit?A Versatile C3 Building Block in Organic Synthesis

Nature employs dihydroxyacetone phosphate (DHAP) as the donor component in various enzyme-catalyzed aldol reactions. Probably the most significant example in this regard is photosynthesis, in which D-glucose, the most widespread natural product, is formed in just a few steps from DHAP. In recent years a number of synthetic equivalents of DHAP have been reported that deserve particular attention, as their applicability in organic synthesis is not limited to (stereoselective) aldol reactions. The power of these reagents has also been demonstrated convincingly in numerous other asymmetric electrophilic alpha-substitution reactions in target-oriented syntheses. Furthermore, the related 1,3-dioxins are useful equivalents of 2-substituted acrolein derivatives.