Clean synthesis initiatives

Application of the deoxygenation process to ff-hydroxysulfides or P-hydroxysulfones results in the clean formation of ra j-olefins through ji-elimination of a thiyl or sulfonyl radical [17]. This mild, radical version of the Julia olefin synthesis, initially described by Lythgoe and Waterhouse [17a], has been applied by two groups in the total synthesis of (+)-pseudomonic acid C. The transformation shown in Scheme 9 is taken from the synthesis by Williams and coworkers [17b]. 

Other aromatic heterocycles undergo Patemo-Btichi reaction with carbonyl compounds, although these reactions have seldom been applied to organic synthesis. For example, thiophene reacts cleanly with benzaldehyde to afford a single exo product in 63% yield87. Pyrroles also react with aldehydes and ketones however, as a result of the lability of the presumed initial cycloadducts, the only products isolated, even with the rigorous exclusion of acid, are the 3-hydroxyalkylpyrroles 200 (equation 7)89. 

The chemist must use the notions of synthetic organic chemistry to initially select the most reasonable experimental conditions, to subsequently perform the whole synthetic route, and to eventually modify the reaction parameters, if this proves necessary, to obtain a reliable, robust, clean, and high-yielding synthesis of the target molecule in solution. Even if the required synthesis is reported in the literature, it would be advisable to check carefully the experimental conditions in order to solve possible issues arising from any reaction before moving to the SPS. 

The anionic ROP of cyclotrisiloxane in THF initiated with lithium trialkylsilanolate or butyl lithium is our system of choice for the controlled synthesis of functionalized polysiloxanes. The polymerization proceeds in a selective way. The undesired reactions of back biting, chain transfer and terminal unit exchange may be eliminated. The initiation is fast and complete, and the quenching of polymerization by triorganochlorosilanes is fast and clean. The polymerization may be performed at low temperatures, which increases its selectivity [4, 5]. In addition, the SiOLi/THF system tolerates many functional groups in the monomer. 

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