Discovery of Annulating Reagents Enabling the One-Step and Highly Stereoselective Synthesis of Cyclopentyl and Cyclohexyl Cores.

The use of the unprecedented annulating reagents methyl N-(tert-butylsulfinyl)-4-chlorobutanimidate and methyl N-(tert-butylsulfinyl)-5-bromopentanimidate enables the diastereoselective preparation of 5- and 6-membered carbocycles bearing three contiguous stereocenters. These synthons undergo cycloaddition with a variety of Michael acceptors to form cyclopentane/cyclohexane rings with excellent stereochemical control, generating only one of the eight possible diastereomers. This novel methodology has enabled the highly enantioselective and high yielding synthesis of novel chemotypes of pharmacological relevance.

A 16-step synthesis of the isoryanodane diterpene (+)-perseanol.

(+)-Perseanol is an isoryanodane diterpene that is isolated from the tropical shrub Persea indica1 and has potent antifeedant and insecticidal properties. It is structurally related to (+)-ryanodine, which is a high-affinity ligand for and modulator of ryanodine receptors-ligand-gated ion channels that are critical for intracellular Ca2+ signalling in most multicellular organisms2. Ryanodine itself modulates ryanodine-receptor-dependent Ca2+ release in many organisms, including mammals; however, preliminary data indicate that ryanodane and isoryanodane congeners that lack the pyrrole-2-carboxylate ester-such as perseanol-may have selective activity in insects3. Here we report a chemical synthesis of (+)-perseanol, which proceeds in 16 steps from commercially available (R)-pulegone. The synthesis involves a two-step annulation process that rapidly assembles the tetracyclic core from readily accessible cyclopentyl building blocks. This work demonstrates how convergent fragment coupling, when combined with strategic oxidation tactics, can enable the concise synthesis of complex and highly oxidized diterpene natural products.

An Oxidative Dearomatization Approach To Prepare the Pentacyclic Core of Ryanodol.

An approach to synthesize the pentacyclic framework of the polyol diterpenoid ryanodol is reported. The ABC tricycle was constructed by a Co-mediated Pauson-Khand reaction, and both radical and anionic cyclization pathways were developed to form the E-ring. In addition, a reaction sequence involving SeO2-mediated enone oxidation and hydroxyl-directed oxy-Michael addition was developed to introduce the A-ring oxidation. The feasibility of forming the bridging D-ring by an oxidative dearomatization was established.

Chemical Synthesis of (+)-Ryanodine and (+)-20-Deoxyspiganthine.

(+)-Ryanodine is a natural product modulator of ryanodine receptors, important intracellular calcium ion channels that play a critical role in signal transduction leading to muscle movement and synaptic transmission. Chemical derivatization of (+)-ryanodine has demonstrated that certain peripheral structural modifications can alter its pharmacology, and that the pyrrole-2-carboxylate ester is critical for high affinity binding to ryanodine receptors. However, the structural variation of available ryanodine analogues has been limited by the challenge of site-specific functionalization of semisynthetic intermediates, such as (+)-ryanodol. Here we report a synthetic strategy that provides access to (+)-ryanodine and the related natural product (+)-20-deoxyspiganthine in 18 and 19 steps, respectively. A key feature of this strategy is the reductive cyclization of an epoxide intermediate that possesses the critical pyrrole-2-carboxylate ester. This approach allows for the direct introduction of this ester in the final stage of the synthesis and provides a framework for the synthesis of previously inaccessible synthetic ryanoids.

Radical Isomerization and Cycloisomerization Initiated by H• Transfer.

Under H2 pressure, Co(II)(dmgBF2)2L2 (L = H2O, THF) generates a low concentration of an H• donor. Transfer of the H• onto an olefin gives a radical that can either (1) transfer an H• back to the metal, generating an isomerized olefin, or (2) add intramolecularly to a double bond, generating a cyclized radical. Transfer of an H• back to the metal from the cyclized radical results in a cycloisomerization. Both outcomes are favored by the low concentration of the cobalt H• donor, whereas hydrogenation and cyclohydrogenation are more likely with other catalysts (when the concentration of the H• donor is high).

Direct generation of oxygen-stabilized radicals by H• transfer from transition metal hydrides.

Transition-metal hydrides generate α-alkoxy radicals by H• transfer to enol ethers. We have measured the rate constant for transfer from CpCr(CO)3H to n-butyl vinyl ether and have examined the chemistry of radicals generated by such transfers. Radicals from appropriate substrates undergo 5-exo cyclization, with higher diastereoselectivity than the analogous all-carbon radicals. From such radicals it is straightforward to make substituted tetrahydrofurans.

Effect of double-bond substituents on the rate of cyclization of α-carbomethoxyhex-5-enyl radicals.

Rate constants have been calculated, and compared with experimental results, for the cyclizations of 1-carbomethoxy-1-methyl-5-hexenyl radicals (2) with various substituents on C6. The calculations have been done by DFT at the B3LYP/6-311++G** level of theory. They show considerable interaction between C5 and the radical centers even in the ground state of all of the radicals 2. Experimentally, the radicals have been generated by H(•) transfer to the corresponding acrylate esters 1 and the yields of cyclized products compared to the calculated rate constants. (The "cyclized products" include those from cyclohydrogenation, 4, and those from cycloisomerization, 9.) Two phenyl substituents on C6 (2i), or a phenyl and a methyl substituent (2g, 2h), increase the rate of cyclization, but a single phenyl substituent on C6 produces a greater increase. The calculations show that the two phenyl substituents are twisted in the transition state for cyclization, while a single phenyl substituent remains flat in that transition state. A methyl substituent on C6 along with a single phenyl causes the phenyl to twist in the transition state and decreases the rate constant for cyclization below that of the H/Ph-substituted 2e, 2f.

Evidence for formation of a Co-H bond from (H2O)2Co(dmgBF2)2 under H2: application to radical cyclizations.

Under H(2), the radical cyclization of appropriate dienes can be catalyzed by cobaloximes. H• can be abstracted from an intermediate (presumably a cobalt hydride) by trityl radicals (Ar(3)C•) or by TEMPO. The rate-determining step in these reactions is the uptake of H(2), which is second order in cobalt and first order in hydrogen; the third-order rate constant is 106(3) M(-2)·s(-1).