elias james corey brought a more formal approach to synthesis design, based on retrosynthetic analysis, for which he won the nobel prize for chemistry in 1990. in this approach, the research is planned backwards from the product, using standard rules. the steps are shown using retrosynthetic arrows (drawn as: =&gt;), which in effect means "is made from". computer programs have been written for designing a synthesis based on sequences of generic "half-reactions".