[poonilization]

Hello, sorry to bother you folks again with my citrinin assignment, i've managed to draw out a complete mechanism for citrinin however i'm having difficulty understanding the labelling study.

Reference: Rachel H. Carter, Mary J. Garson and James Staunton,  J.C.S. Chem. Comm., 1979, 1097-1098   (couldn’t find volume number)
image of reference paper : http://i.gyazo.com/a7a75adc48410af1883ce80d00026c0d.png

Ok so the reaction scheme on the left shows 5 acetate units condensing to form a pentaketide which goes through a series of reactions and forms Citrinin, the molecule labelled (1).
Molecules labelled (2), (3) and (4) are all intermediates.

The part i have highlighted states that methylation of the polyketide chain occurs before the chain condenses to form a ring. How did they come to that conclusion from the labelling experiment as i'm having difficulty understanding some of the terminolgy used. What is an incorporation experiment? what is % molar activity of acetic acid and methylamin^b? What does citrinin incorporated 0.7% of the activity of molecule (3) mean? and molecule (4) gave 0.05% incorporation?

I understand molecules (3) and (4) are labelled with ¹⁴C at carbon 9 and i'm lost after that.

Also how can molecule (3) even exist, it is stated that methylation occurs before the ring forms, so how can a ring where methylation hasn't happened and R=H exist?

Can anyone help break down the results of the labelling experiment please?