No abstract

Stereoselective synthesis of the C1-C29 part of amphidinol 3 (AM3) was achieved. The C1-C20 part was assembled from three building blocks via regioselective cross metathesis to form the C4-C5 double bond and addition of an alkenyllithium and a lithium acetylide to two Weinreb amides followed by asymmetric reduction to form the C9-C10 and C14-C15 bonds, respectively. The C21-C29 part was synthesized via successive cross metathesis and oxa-Michael addition sequence to construct the 1,3-diol system at C25 and C27 and Brown asymmetric crotylation to introduce the stereogenic centers at C23 and C24. Coupling of the C1-C20 and C21-C29 parts was achieved by Julia-Kocienski olefination and regio- and stereoselective dihydroxylation of the C20-C21 double bond in the presence of the C4-C5 and C8-C9 double bonds to afford the C1-C29 part of AM3.

Cross‐sectional transmission electron microscopy (TEM) was performed to observe the behavior of threading dislocations (TDs) in a heterostructure of a GaN/AlGaN layer grown on IL‐AlN/GaN/LT‐GaN/α‐Al2O3(0001), where IL‐AlN and LT‐GaN are an intermediate layer of AlN and a low‐temperature buffer layer of GaN, respectively. TDs of ( a + c)‐type, or TDs with a Burgers vector a + c, are formed at the LT‐GaN layer and propagate upwards as far as the top surface, penetrating the AlGaN layer. A number of a‐type TDs are generated at the IL, and many of these are annihilated within about 100 nm above the GaN/AlGaN interface. Quick stress relief in the GaN layer on AlGaN is due to the annihilation of a‐type TDs.