The mechanism of acetylene bromoboration in neat boron tribromide was studied carefully by means of experiment and theory. Besides the syn-addition mechanism through a four-center transition state, radical and polar anti-addition mechanisms are postulated, both triggered by HBr, which is evidenced also to take part in the Z/E isomerization of the product. The proposed mechanism is well supported by ab initio calculations at the MP2/6-31+G* level with Ahlrichs' SVP all-electron basis for Br. Implicit solvation in CH 2 Cl 2 has been included using the PCM and/or SMD continuum solvent models. Comparative case studies have been performed involving the B3LYP/6-31+G* with Ahlrichs' SVP for Br and MP2/Def2TZVPP levels. The mechanistic studies resulted in development of a procedure for stereoselective bromoboration of acetylene yielding E/Z mixtures of dibromo(bromovinyl)borane with the Z-isomer as a major product (up to 85%). Transformation to the corresponding pinacol and neopentyl glycol boronates and stereoselective decomposition of their E-isomer provided pure (Z)-(2-bromovinyl)boronates in 57−60% overall yield. Their reactivity in a Negishi cross-coupling reaction was tested. An example of the one-pot reaction sequence of Negishi and Suzuki− Miyaura cross-couplings for synthesis of combretastatin A4 is also presented.

We report a general synthetic entry to dihydrooxepine-spiroisoxazoline (DOSI) natural products that culminated in the first racemic total synthesis of psammaplysin A. For the synthesis of the unique spirocyclic fragment we employed a strategy that features two key transformations: (1) a diastereoselective Henry reaction/cyclization sequence to access the C7 hydroxylated isoxazoline scaffold in one step and (2) a regioselective Baeyer−Villiger ring expansion to install the fully substituted dihydrooxepine and avoid the risk of a previously observed oxepine-arene oxide rearrangement. The overall synthesis proceeds in 13 steps from an inexpensive starting material.

Here, we report a general synthetic entry to dihydrooxepine-spiroisoxazoline (DOSI) natural products that culminated in the first total synthesis of psammaplysin A. For the synthesis of the unique spirocyclic fragment we employed a strategy that features two key transformations: (1) The use of a diastereoselective Henry reaction/cyclization sequence granted access to the C7 hydroxylated isoxazoline scaffold in one step. (2) A regioselective Baeyer–Villiger ring expansion enabled selective installation of the fully substituted dihydrooxepine and avoided the risk of a previously observed oxepine-arene oxide rearrangement. The overall synthesis proceeds in 13 steps from inexpensive starting material.

Here, we report a general synthetic entry to dihydrooxepine-spiroisoxazoline (DOSI) natural products that culminated in the first total synthesis of psammaplysin A. For the synthesis of the unique spirocyclic fragment we employed a strategy that features two key transformations: (1) The use of a diastereoselective Henry reaction/cyclization sequence granted access to the C7 hydroxylated isoxazoline scaffold in one step. (2) A regioselective Baeyer–Villiger ring expansion enabled selective installation of the fully substituted dihydrooxepine and avoided the risk of a previously observed oxepine-arene oxide rearrangement. The overall synthesis proceeds in 13 steps from inexpensive starting material.