Highly efficient and selective synthesis of conjugated triynes and higher oligoynes of biological and materials chemical interest via palladium-catalyzed alkynyl-alkenyl coupling

Iteration of a Pd-catalyzed reaction of alkynyl- and oligoynylzincs with (E)-ICH=CHCl followed by metalation-termination with electrophiles(E) has provided a linear route to conjugated tri- and tetraynes, and Pd-catalyzed monoalkynylation of 1,1-dibromoenynes accompanied by dehydrobromination has provided a convergent route to conjugated tri- , tetra- , and pentaynes. Both display unprecedented high efficiency and selectivity. [reaction: see text]     

Highly stereoselective total synthesis of fully hydroxy-protected mycolactones A and B and their stereoisomerization upon deprotection

Unprecedentedly efficient and highly (≥98 %) stereoselective syntheses of mycolactones A and B side chains relied heavily on Pd‐catalyzed alkenylation (Negishi version) and were completed in 11 longest linear steps from ethyl (S)‐3‐hydroxybutyrate in 12 % and 11 % overall yield, respectively, roughly corresponding to an average of 82 % yield per step. The synthesis of mycolactone core was realized by using Pd‐catalyzed alkenyl? allyl coupling and an epoxide‐opening reaction with a trialkylalkenylaluminate as key steps. Fully hydroxy‐protected mycolactones A and B of ≥98 % isomeric purity were synthesized successfully for the first time. However, unexpected 4:3–5:4 inseparable mixtures of mycolactones A and B were obtained upon deprotection.     

High Resolution FTIR Study on NH(2)Cl Molecule: Inversion and Anharmonicity

The high-resolution FTIR absorption spectra of the NH(2)Cl molecule have been measured in the region 500-3600 cm(-1). The rotational structures of all the fundamental bands and a number of overtone and combination bands have been analyzed. Every rotational line of the observed bands except the NH(2) twisting and stretching fundamental bands splits into two components due to the amino inversion motion. The vibrational anharmonicity and the direction of the transition moments have been studied with the help of B3LYP/6-31++G(**) calculations. The amount of inversion splitting depends on the molecular rotation, and the J,K-dependence of the inversion splitting has been determined. Copyright 2001 Academic Press.     

A highly efficient, selective, and general method for the synthesis of conjugated (all-E)-oligoenes of the (CH=CH)n type via iterative hydrozirconation-palladium-catalyzed cross-coupling

[reaction: see text] A linear iterative method for the construction of (all-E)-oligoenes of the (CH=CH)n type via hydrozirconation-palladium-catalyzed cross-coupling with (E)-1-bromo-4-trimethylsilyl-1-buten-3-yne is described. This method promises to provide an efficient, selective, and general route to oligoene macrolide antibiotics and other related natural products.     

Highly stereo- and regioselective synthesis of (Z)-trisubstituted alkenes via 1-bromo-1-alkyne hydroboration-migratory insertion-Zn-promoted iodinolysis and Pd-catalyzed organozinc cross-coupling

Hydroboration of 1-bromo-1-alkynes with dibromoborane followed by addition of 3 or 4 equiv of Me2Zn provides an efficient and selective route to (Z)-2-alkenyldimethylboranes (3) or (Z)-2-alkenylmethylzincs (4), respectively, which have been successfully applied to one-pot Suzuki (B-I) or Negishi (Zn-I) coupling in some less demanding cases. However, in more demanding cross-coupling reactions, only the use of either (Z)-2-alkenyliodides (5) or the alkenylzincs prepared from lithiation and then zincation of 5 proves to be highly satisfactory (Zn-II or Zn-III protocol). On the contrary, the corresponding organoboron coupling under B-II or B-III protocol appears to be less satisfactory. Preliminary studies indicate that certain substituents proximal to trisubstituted alkenes seriously affect the course of the desired alkenylboron cross-coupling.     

Cyclization reactions of ω-tosyloxy-1-alkynyl- and ω-tosyloxy-1-alkenylborates and their ω-halo analogues

The reaction of trialkylboranes with ω-tosyloxy-1-lithio-1-alkynes can induce transfer of an alkyl group from the boron atom to the alkynyl carbon atom with concomitant formation of four- through six-membered carbocycles via intramolecular displacement of the ω-tosyloxy group. The stereoselectivity of the reaction, however, is low (anti/syn≃1.6–1.7). The corresponding reaction of ω-halo- or ω-tosyloxy-1-alkenylborates also gives exocyclic alkenes via 1,2-migration-cyclization followed by dehydroboration. In the cases of cyclopropanation, cyclopropylcarbinyl-to-homopropargyl rearrangement rather than dehydroboration takes place. Diphenylzirconocene reacts similarly with 6-lithio-5-hexynyl tosylate to give phenylmethylenecyclopentane in 45% yield. On the other hand, attempts to induce a similar migration with phenyl derivatives of Y, V, Cr, and Mn have led to < 5–10% yields of the same cyclization product.     

Catalytic, efficient, and syn-selective construction of deoxypolypropionates and other chiral compounds via Zr-catalyzed asymmetric carboalumination of allyl alcohol

An efficient, syn-selective, all catalytic asymmetric protocol for the synthesis of alpha,omega-diheterofunctional deoxypolypropionates via Zr-catalyzed asymmetric carboalumination (ZACA reaction) was developed. The success of the method critically hinges on the one-pot conversion of unprotected allyl alcohol into TBS-protected (R)- or (S)-3-iodo-2-methyl-1-propanol (1) of 91% enantiomeric purity in 82% yield via (i) Zr-catalyzed asymmetric methylalumination, (ii) iodination, and (iii) protection with TBSCl. After zincation of 1, its Pd-catalyzed cross-coupling with various organic halides can give various organic derivatives, including 3 and 4, which can serve as key intermediates for efficient and selective syntheses of deoxypolypropionates, such as doliculide and 2,4,6,8-tetramethyldecanoic acid, and other chiral compounds, such as callystatin A. Selective monocarboalumination of 1,4-pentadiene (5 equiv) gave, after oxidation, the expected product 5, but it was 0% ee. A plausible mechanism for racemization has been proposed and experimentally supported.     

A convenient and genuine equivalent to HZrCp2Cl generated in situ from ZrCp2Cl2-DIBAL-H

[reaction: see text] Slow addition of 1 equiv of (i)Bu2AlH to ZrCp2Cl2 in THF provides a convenient route to either HZrCp2Cl-(i)Bu2AlCl (Reagent I) or HZrCp2Cl (Reagents II and III). The latter represents a highly convenient route to genuine HZrCp2Cl, while Reagent I is useful for regio- and stereoselective conversion of 1- and 2-alkynes into (E)-1-iodo-1-alkenes and (E)-2-iodo-2-alkenes, respectively.     

Catalytic enantioselective synthesis of chiral organic compounds of ultra-high purity of >99% ee

Shortly after the discovery of Zr-catalyzed carboalumination of alkynes in 1978, we sought expansion of the scope of this reaction so as to develop its alkene version for catalytic asymmetric C–C bond formation, namely the ZACA (Zr-catalyzed asymmetric carboalumination of alkenes). However, this seemingly easy task proved to be quite challenging. The ZACA reaction was finally discovered in 1995 by suppressing three competitive side reactions, i.e., (i) cyclic carbometalation, (ii) β-H transfer hydrometalation, and (iii) alkene polymerization. The ZACA reaction has been used to significantly modernize and improve syntheses of various natural products including deoxypolypropionates and isoprenoids. This review focuses on our recent progress on the development of ZACA–lipase-catalyzed acetylation–transition metal-catalyzed cross-coupling processes for highly efficient and enantioselective syntheses of a wide range of chiral organic compounds with ultra-high enantiomeric purities.     

Stereo- and regioselective routes to allylic silanes

The Ni- or Pd-catalyzed reaction of alkenyl iodides with Me₃SiCH₂Mgcl provides various types of allytrimethylsilanes in excellent yields in a highly stereo- and regioselective manner, while the Zr-catalyzed carboalumination of Me₃SiCH₂CCH followed by replacement of Al with carbon groups by known reactions produces allylsilanes represented by $\text{2}$.