Big banks not interested in your green business? No problem. A small investment bank whose employees have dozens of years of Wall Street experience is now on the scene, specializing in green business.

Greentech Capital Advisors is led by Jeffrey McDermott, who left Swiss banking giant UBS two years ago. He was one of three co-heads of investment banking there.

Greentech offers financial advisory services, including buy-side and sell-side merger and acquisition, exclusive sale transactions, restructurings, private placements, and project finance advice, exclusively in the alternative energy and cleantech sectors.

This new boutique “green” bank may have come along at just the right time.

A United Nations report says new financial investment in the sustainable energy sector fell by 53% in the first quarter of 2009, compared to the same period the previous year. That’s the lowest quarterly investment for three years.

While the world financial crisis has crimped green investing, it’s produced a wealth of experienced employees for Greentech. So the new investment bank has a seasoned staff.

Greentech has one deal under its belt – the sale of Solar Integrated Technologies to Energy Conversion Devices for $16.3 million in June.

On the one hand, it’s just one sale handled by Greentech. On the other hand, it’s good news for smaller companies in the green economy. Try getting one of the mega-banks interested in a deal under $100 million.

Greentech is attracting some notice. A New York Times article notes it’s had more than 350 meetings with potential clients and advised a number of them on acquiring green companies.