 In the final step the BOC group is removed (HCl) and phosphoric acid added to objective 16.An approach published in 2007  like Corey's starts by an asymmetric Diels-Alder reaction this time with starting materials pyridine and acrolein.Pyridine (1) is reduced with sodium borohydride in presence of benzyl chloroformate to the Cbz protected dihydropyridine 2.