Target preparation retrosynthesis

Scheme 8.1 Retrosynthesis in synthetic organic chemistry to determine suitable synthons and hence prepare the target molecule from suitable synthetic equivalents. The retrosynthetic steps are indicated by open arrows.

Since the targeted streptazolin-related natural products have a C2-unit at the C6-position, we first prepared the 2-oxazolone-alkyne derivative 82 with the simplest C2-unit, an ethyl group, at the triple bond terminus to not only identify suitable ring-closing conditions, but also to determine the level of stereoselectivity that could be expected in the intramolecular Pauson-Khand reaction. Thus, the 2-oxazolone-alkyne derivative 82, required for the intramolecular Pauson-Khand reaction in our retrosynthesis, was easily prepared from the known alcohol 78 by conventional means, as shown in Scheme 18. Oxidation of 78 was... 

One possible retrosynthesis of a 1,6-dicarbonyl involves a functional group interconversion to a cyclohexene. The cyclohexene derivative can be prepared in a variety of ways (e.g., by a Diels-Alder reaction), so the retrosynthesis can continue from there. The synthesis of target molecules containing six-membered rings will be discussed in a later section. 

Each of the following target molecules can be prepared via an intramolecular Diels-Alder reaction. Determine the required disconnection/ retrosynthesis for each, paying close attention to the stereochemistry of the TM. 

---