Selective synthesis of (S)-1-methylbutyl (E)-2,4-dimethyl-2-pentanoate, and aggregation pheromone component of Rhyzopertha dominica, and of (E)-2-butyl-2-octenal, an alarm pheromone component of Oecophylla longinoda, via 2-substituted 1-silyl-1-stannylethenes