The structure of dipyrroles and indole formation from pyrroles : stereochemistry of linalool. --.

THE STRUCTUBE OF DIPYRROLES AND INDOLE FORMATION FROM PYRROLES: From the examples cited in the historical section it was seen that pyrroles reacted with themselves to form dimeric addition products. [...] Pyrroles seldom form salts with acids, and methiodides are practically unknown. Since dipyrroles form salts with one equivalent of acid, at least one of the nitrogen atoms must be different, hence one of the characteristic pyrrole linkages has disappeared. [...] Hydrogen atoms undergo a tautomeric shift is pyrroles, in that the usual reagents give derivatives of either possible form, i.e. HN02 gives ß-nitroso derivatives only, even with pyrrole itself; and acylating agents usually attack the carbon alpha to the nitrogen atom. The Grignard reagent should show the presence of two active hydrogen atoms, but the use of this reagent as a method for determining structure in pyrrole series is known to be unreliable. [...] From the examples cited, it seems evident that like any substance of the type HA pyrroles should add to the conjugated system of a second reactant. When there is nothing else present for the pyrrole to combine with, it should add to itself for it contains the necessary conjugated system. Since pyrroles can be kept unchanged for long periods of time, this tendency cannot be very great. [...]STEREOCHEMISTRY OF LINALOOL: For a long time it has been known that optically active straight chain secondary alcohols can be converted into optically active secondary halides. [...] More recently however Hughes, Ingold, and Masterman (3) have prepared halides of straight chain aliphatic secondary carbinols such as methylhexylcarbinol under a variety of conditions such as saturation with the dry gaseous hydrogen halides and treatment with thionyl halides. They found that there was always some racemization, for the activity of their halides varied with the method of conversion. The amounts of racemization varied from 50% to as little as 3%. [...] In order to avoid this complex type so bound up with possible free radical formation, an investigation was made of the simple optically active tertiary alcohols. As there were no methods for resolving them, natural substances had to be considered. Linalool seemed to be well adapted to this work as it was readily available in an optically active form and could be reduced to the simplest type of optically active tertiary-carbinol possible [...] l-linalool was first reduced to a saturated compound before conversion into its halide derivative. Complete reduction was possible in glacial acetic acid using Adams platinum catalyst. Several other methods were tried, but this was the only one which gave the desired result. [...