An Improved Synthetic Preparation of 3-Butenal

Differences in the torsional anharmonicity between reactant and transition state: the case of 3-butenal + H abstraction reactions

Thermal rate coefficients for the hydrogen-abstraction reactions of 3-butenal by a hydrogen atom were obtained applying multipath canonical variational theory with small-curvature tunneling (MP-CVT/SCT). Torsional anharmonicity due to the hindered rotors was taken into account by calculating the rovibrational partition function using the extended two-dimensional torsional (E2DT) method. For comparison, rovibrational partition functions were also estimated using the multistructural method with torsional anharmonicity based on a coupled torsional potential (MS-T(C)). By contrast, with (E)-2-butenal reactions, the abstraction reactions of 3-butenal proceed via five reaction channels (R1)-(R5). In a conformational search, 45 distinguishable structures of transition states were found, including enantiomers, which were separated into six conformational reaction channels (CRCs). The individual reactive paths were constructed, the recrossing and semiclassical transmission coefficients estimated, and the multipath rate constants were obtained. High torsional barriers between the wells of CRC2/CRC6 indicate a harmonic behavior. Consequently, a difference between the torsional anharmonicity of 3-butenal and the transition states is responsible for the increase in the thermal rate constants for channel (R2). Analysis of the contributions of each conformer of the transition state shows an important contribution of the high-energy rotamers in the total flux of (R1)-(R5). After fitting the rate constants in a four-parameter equation, the activation energy estimation showed a strong temperature dependence.

Total synthesis of (+)-herboxidiene/GEX 1A

An efficient synthesis of herboxidiene is granted from highly stereoselective aldol reactions from two lactate-derived ketones and an oxa-Michael cyclization.

Synthetic studies toward the brasilinolides: controlled assembly of a protected C1–C38 polyol based on fragment union by complex aldol reactions

A modular strategy for the synthesis of the immunosuppressant macrolide brasilinolide A was adopted based on coupling suitable northern and southern fragments.

A diastereoselective oxa-Pictet-Spengler-based strategy for (+)-frenolicin B and epi-(+)-frenolicin B synthesis

An efficient diastereoselective oxa-Pictet-Spengler reaction strategy was developed to construct benzoisochroman diastereomers. The utility of the reaction was demonstrated in the context of both the total synthesis of naturally occurring pyranonaphthoquinones (+)-frenolicin B and epi-(+)-frenolicin B as well as a range of frenolicin precursor analogs. The method is versatile and offers exquisite stereocontrol and, as such, offers a synthetic advance for the synthesis of pyranonaphthoquinone analogs.

Enantioselective synthesis of the C1-C6 and C7-C23 fragments of the proposed structure of iriomoteolide 1a

Synthesis of the C1-C6 and C7-C23 fragments of the proposed structure of iriomoteolide 1a has been accomplished. Key steps include a cross metathesis to form the C15-C16 E olefin and a chelation controlled Grignard addition to form the tertiary alcohol at C14. Notably, 7 of the 9 stereocenters of the proposed structure have been set using various aldol reactions employing metallo enolates of thiazolidinethiones.

Total synthesis of the proposed structure of aldingenin B

The first enantioselective total synthesis of the proposed structure of aldingenin B is reported in 16 steps from known compounds. The stereochemistry at C5 and C6 were established by an asymmetric acetal aldol. Following a ring-closing metathesis, a selective, substrate-controlled hydrogen bond-mediated dihydroxylation provided control of the C2 and C3 stereocenters. Discrepancies in the spectroscopic data of the synthetic and natural material led to the conclusion that the structure of the natural sample was misassigned.

Total synthesis of (+)-herboxidiene from two chiral lactate-derived ketones

A substrate-controlled synthesis of (+)-herboxidiene from two lactate-derived chiral ketones is described. Remarkably, most of the carbon backbone was constructed through highly stereoselective titanium-mediated aldol reactions and an Ireland-Claisen rearrangement. Furthermore, an oxa-Michael cyclization and a high-yield Suzuki coupling were used to assemble the pyran ring and the diene moiety respectively.

Enantioselective total synthesis of spirofungins A and B

The enantioselective total synthesis of spirofungins A (1) and B (2) is reported in 14 steps over the longest linear sequence. Key steps include the use of thiazolidinethione-mediated aldol reactions to assemble the major fragments and installation of the C1-C6 side chain using a cross metathesis reaction.

Total synthesis of (-)-reidispongiolide A, an actin-targeting macrolide isolated from the marine sponge Reidispongia coerulea

A stereocontrolled total synthesis of the microfilament-destabilizing cytotoxic macrolide (-)-reidispongiolide A, isolated from the New Caledonian marine sponge Reidispongia coerulea, is described. This synthesis utilizes a convergent aldol-based strategy to construct the 26-membered macrolactone, followed by the late-stage coupling of a derived aldehyde with an N-vinylformamide-containing ketone subunit to install the full side chain. Two alternative routes were examined for the introduction of the 2E,4E-dienoate region, and a complex Mukaiyama aldol coupling was used to connect the northern and southern hemispheres to install the C13 stereocenter. This constitutes the first chemical synthesis of any member of the reidispongiolide/sphinxolide family of marine macrolides and unequivocally establishes the relative and absolute configuration.

A New Nucleophilic Addition/Ring-Closure Sequence. Enantioselective Synthesis of 3-Deoxy-8-oxatropanes

[reaction: see text] A study of new nucleophilic addition/ring-closure (NARC) sequences has resulted in the development of a stereoselective synthetic route to 3-deoxy-8-oxatropanes. The new sequences consisted of either a syn or anti aldol addition, employing an omega-alkenoyl sultam, followed by two-step bicyclic ring construction involving, consecutively, ring-closing metathesis and intramolecular oxymercuration.

Direct Vinylogous Mannich-Type Reactions via Ring Opening and Rearrangement of Vinyloxiranes

[reaction: see text] The first synthetic application of 2-methyl-2-vinyloxirane as a masked dienolate has been successfully demonstrated in the direct vinylogous Mannich-type reaction with an alpha-imino ester as an electrophile. The Mannich adduct was a useful intermediate en route to cis-5-substituted pipecolinic acid ethyl ester under simple hydrogenation conditions.

Enantio- and diastereoselective synthesis of cyclic beta-hydroxy allylsilanes via sequential aldehyde gamma-silylallylboration and ring-closing metathesis reactions

[reaction: see text] Highly enantioenriched cyclic allylsilanes are prepared via stereoselective gamma-silylallylboration reactions of beta- or gamma-unsaturated aldehydes followed by ring-closing metathesis.

Formal Total Synthesis of (+)-Trehazolin. Application of an Asymmetric Aldol−Olefin Metathesis Approach to the Synthesis of Functionalized Cyclopentenes

An asymmetric synthesis of the aminocyclopentitol pseudosugar of trehazolin has been completed. The synthesis hinges on an asymmetric aldol-ring closing metathesis strategy to construct the five-membered ring with control of both the relative and absolute stereochemistry.

An aldol approach to the synthesis of the EF fragment of spongistatin 1

A synthesis of the C29-C51 fragment of spongistatin 1, containing the E and F rings, has been completed. The approach relies on four diastereoselective aldol additions and an asymmetric glycolate alkylation to establish eight of the eleven stereogenic centers. The intact chlorodiene side chain was appended by a Lewis acid catalyzed addition of an allylstannane to an epoxy enol ether.

Use of gamma-carboxy-alpha,beta-unsaturated aldehydes as synthetic equivalents of beta,gamma-unsaturated aldehydes in a novel stereoselective approach to diketides

[reactions: see text] A two-step aldol/Pd-catalyzed formate reduction sequence allows for convenient, stereoselective access to products of great potential in synthetic approaches to polyketides. A novel diastereoselective Pd-catalyzed formate reduction of allylic acetates is reported.