Key Step to Hirsutene

A key step in the total synthesis of ( )-hirsutene was the acid catalysed (p-TsOH/CH 2CI2/RT) cyclisation of 1 to 2, which proceeded in 95% yield and gave a 10 1 mixture of epimers at C4 (the major epimer was 2). 

Give a mechanism for this transformation which accounts for the observed stereochemistry. 

---